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http://www.mdpi.com/2073-4344/5/3/1399
Platinum (Pt) nanoparticles are deposited on a hybrid support (C-MnO2) according to a polyol method. The home-made catalyst, resulted as Pt/C-MnxO1+x, is compared with two different commercial platinum based materials (Pt/C and PtRu/C). The synthesized catalyst is characterized by means of FESEM, XRD, ICP-MS, XPS and μRS analyses. MnO2 is synthesized and deposited over a commercial grade of carbon (Vulcan XC72) by facile reduction of potassium permanganate in acidic solution. Pt nanoparticles are synthesized on the hybrid support by a polyol thermal assisted method (microwave irradiation), followed by an annealing at 600 °C. The obtained catalyst displays a support constituted by a mixture of manganese oxides (Mn2O3 and Mn3O4) with a Pt loading of 19 wt. %. The electro-catalytic activity towards MOR is assessed by RDE in acid conditions (0.5 M H2SO4), evaluating the ability to oxidize methanol in 1 M concentration. The synthesized Pt/C-MnxO1+x catalyst shows good activity as well as good stability compared to the commercial Pt/C based catalyst.Catalysts2015-07-3053Article10.3390/catal5031399139914162073-43442015-07-30doi: 10.3390/catal5031399Alessandro VidelaLuigi OsmieriReza EsfahaniJuqin ZengCarlotta FranciaStefania Specchiahttp://www.mdpi.com/2073-4344/5/3/1388
The newly proposed mimetic underpotential deposition (MUPD) technique was extended to modify Pd surfaces with Sb through immersing a Pd film electrode or dispersing Pd/C powder in a Sb(III)-containing solution blended with ascorbic acid (AA). The introduction of AA shifts down the open circuit potential of Pd substrate available to achieve suitable Sb modification. The electrocatalytic activity and long-term stability towards HCOOH electrooxidation of the Sb modified Pd surfaces (film electrode or powder catalyst) by MUPD is superior than that of unmodified Pd and Sb modified Pd surfaces by conventional UPD method. The enhancement of electrocatalytic performance is due to the third body effect and electronic effect, as well as bi-functional mechanism induced by Sb modification which result in increased resistance against CO poisoning.Catalysts2015-07-2853Article10.3390/catal5031388138813982073-43442015-07-28doi: 10.3390/catal5031388Long-Long WangXiao-Lu CaoYa-Jun WangQiao-Xia Lihttp://www.mdpi.com/2073-4344/5/3/1375
Pd core nanoparticles less than 5 nm in mean size were prepared on carbon black (CB) without any stabilizer by using palladium acetate as a precursor and CO as a reducing agent, and then used for preparing Pd core/Pt shell nanoparticles-loaded CB (Pt/Pd/CB). The mean size of Pd nanoparticles could be controlled by the concentration of palladium acetate and the CO bubbling time. The cyclic voltammograms of two Pd nanoparticles-loaded CB (Pd4.2/CB, Pd3.3/CB) electrodes whose mean size was 4.2 and 3.3 nm, respectively, had characteristics similar to a Pt electrode after the formation of a Pt monolayer shell, suggesting that the Pd core nanoparticles were almost covered with the Pt monolayer shell. The oxygen reduction reaction (ORR) on both Pt/Pd/CB proceeded in 4-electron reduction mechanism. Both Pt/Pd/CB electrodes was ca. 1.5 times higher in ORR activity per electrochemical surface area of Pt (specific activity, SA) than the commercial Pt nanoparticles-loaded CB (Tanaka Kikinzoku Kogyo, Pt/CB-TKK) electrode, and the Pt/Pd3.3/CB electrode had higher SA than the Pt/Pd4.2/CB electrode. The ORR activity per unit mass of Pt for both Pt/Pd/CB electrodes was 5.0 and 5.5 times as high as that for the Pt/CB-TKK electrode, respectively. The durability of both Pt/Pd/CB electrodes was comparable to that of Pt/CB-TKK.Catalysts2015-07-2853Article10.3390/catal5031375137513872073-43442015-07-28doi: 10.3390/catal5031375Hiroshi InoueRyotaro SakaiTaiki KuwaharaMasanobu ChikuEiji Higuchihttp://www.mdpi.com/2073-4344/5/3/1352
A novel, solvent-deficient precipitation (SDP) method for catalyst preparation in general and for preparation of iron FT catalysts in particular is reported. Eight catalysts using a 23 factorial design of experiments to identify the key preparation variables were prepared. The catalysts were characterized by electron microprobe, N2 adsorption, TEM, XRD, and ICP. Results show that the morphology of the catalysts, i.e., surface area, pore volume, pore size distribution, crystallite sizes, and promoter distribution are significantly influenced by (1) whether or not the precursor catalyst is washed, (2) the promoter addition step, and (3) the drying condition (temperature). Consequently, the activity, selectivity, and stability of the catalysts determined from fixed-bed testing are also affected by these three variables. Unwashed catalysts prepared by a one-step method and dried at 100 °C produced the most active catalysts for FT synthesis. The catalysts of this study prepared by SDP compared favorably in activity, productivity, and stability with Fe FT catalysts reported in the literature. It is believed that this facile SDP approach has promise for development of future FT catalysts, and also offers a potential alternate route for the preparation of other catalysts for various other applications.Catalysts2015-07-2453Article10.3390/catal5031352135213742073-43442015-07-24doi: 10.3390/catal5031352Kyle BrunnerBaiyu HuangBrian WoodfieldWilliam Heckerhttp://www.mdpi.com/2073-4344/5/3/1333
The effect of ZIF-8 crystal size on the morphology and performance of Fe–N–C catalysts synthesized via the pyrolysis of a ferrous salt, phenanthroline and the metal-organic framework ZIF-8 is investigated in detail. Various ZIF-8 samples with average crystal size ranging from 100 to 1600 nm were prepared. The process parameters allowing a templating effect after argon pyrolysis were investigated. It is shown that the milling speed, used to prepare catalyst precursors, and the heating mode, used for pyrolysis, are critical factors for templating nano-ZIFs into nano-sized Fe–N–C particles with open porosity. Templating could be achieved when combining a reduced milling speed with a ramped heating mode. For templated Fe–N–C materials, the performance and activity improved with decreased ZIF-8 crystal size. With the Fe–N–C catalyst templated from the smallest ZIF-8 crystals, the current densities in H2/O2 polymer electrolyte fuel cell at 0.5 V reached ca. 900 mA cm−2, compared to only ca. 450 mA cm−2 with our previous approach. This templating process opens the path to a morphological control of Fe–N–C catalysts derived from metal-organic frameworks which, when combined with the versatility of the coordination chemistry of such materials, offers a platform for the rational design of optimized Metal–N–C catalysts.Catalysts2015-07-2453Article10.3390/catal5031333133313512073-43442015-07-24doi: 10.3390/catal5031333Vanessa ArmelJulien HannauerFrédéric Jaouenhttp://www.mdpi.com/2073-4344/5/3/1321
The inadequate activity and stability of Pt as a cathode catalyst under the severe operation conditions are the critical problems facing the application of the proton exchange membrane fuel cell (PEMFC). Here we report on a novel route to synthesize highly active and stable oxygen reduction catalysts by depositing Pt monolayer on a nitrided alloy core. The prepared PtMLPdNiN/C catalyst retains 89% of the initial electrochemical surface area after 50,000 cycles between potentials 0.6 and 1.0 V. By correlating electron energy-loss spectroscopy and X-ray absorption spectroscopy analyses with electrochemical measurements, we found that the significant improvement of stability of the PtMLPdNiN/C catalyst is caused by nitrogen doping while reducing the total precious metal loading.Catalysts2015-07-2253Article10.3390/catal5031321132113322073-43442015-07-22doi: 10.3390/catal5031321Jue HuKurian KuttiyielKotaro SasakiDong SuTae-Hyun YangGu-Gon ParkChengxu ZhangGuangyu ChenRadoslav Adzichttp://www.mdpi.com/2073-4344/5/3/1306
Cerium oxide (CeO2) was prepared using a controlled-precipitation method under microgravity at the International Space Station (ISS). For comparison, ceria was also synthesized under normal-gravity conditions (referred as control). The Brunauer-Emmett-Teller (BET) surface area, pore volume and pore size analysis results indicated that the ceria particles grown in space had lower surface area and pore volume compared to the control samples. Furthermore, the space samples had a broader pore size distribution ranging from 30–600 Å, whereas the control samples consisted of pore sizes from 30–50 Å range. Structural information of the ceria particles were obtained using TEM and XRD. Based on the TEM images, it was confirmed that the space samples were predominantly nano-rods, on the other hand, only nano-polyhedra particles were seen in the control ceria samples. The average particle size was larger for ceria samples synthesized in space. XRD results showed higher crystallinity as well as larger mean crystal size for the space samples. The effect of sodium hydroxide concentration on synthesis of ceria was also examined using 1 M and 3 M solutions. It was found that the control samples, prepared in 1 M and 3 M sodium hydroxide solutions, did not show a significant difference between the two. However, when the ceria samples were prepared in a more basic medium (3 M) under microgravity, a decrease in the particle size of the nano-rods and appearances of nano-polyhedra and spheres were observed.Catalysts2015-07-2053Article10.3390/catal5031306130613202073-43442015-07-20doi: 10.3390/catal5031306Ilgaz SoykalHyuntae SohnBurcu BayramPreshit GawadeMichael SnyderStephen LevineHayrani OzUmit Ozkanhttp://www.mdpi.com/2073-4344/5/3/1304
People like to distill complicated phenomenon into easily digestible numbers. [...]Catalysts2015-07-1753Editorial10.3390/catal5031304130413052073-43442015-07-17doi: 10.3390/catal5031304Keith Hohnhttp://www.mdpi.com/2073-4344/5/3/1289
In order to develop noble-metal- and carbon-free cathodes, titanium-niobium oxides were prepared as active materials for oxide-based cathodes and the factors affecting the oxygen reduction reaction (ORR) activity were evaluated. The high concentration sol-gel method was employed to prepare the precursor. Heat treatment in Ar containing 4% H2 at 700–900 °C was effective for conferring ORR activity to the oxide. Notably, the onset potential for the ORR of the catalyst prepared at 700 °C was approximately 1.0 V vs. RHE, resulting in high quality active sites for the ORR. X-ray (diffraction and photoelectron spectroscopic) analyses and ionization potential measurements suggested that localized electronic energy levels were produced via heat treatment under reductive atmosphere. Adsorption of oxygen molecules on the oxide may be governed by the localized electronic energy levels produced by the valence changes induced by substitutional metal ions and/or oxygen vacancies.Catalysts2015-07-1753Article10.3390/catal5031289128913032073-43442015-07-17doi: 10.3390/catal5031289Akimitsu IshiharaYuko TamuraMitsuharu ChisakaYoshiro OhgiYuji KohnoKoichi MatsuzawaShigenori MitsushimaKen-ichiro Otahttp://www.mdpi.com/2073-4344/5/3/1275
A facile, rapid and green method based on potentiostatic electrodeposition is developed to synthesize a novel H3PMo12O40-Pt/reduced graphene oxide (denoted as PMo12-Pt/rGO) composite. The as-prepared PMo12-Pt/rGO is characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results reveal that graphene oxide (GO) is reduced to the rGO by electrochemical method and POMs clusters are successfully located on the rGO as the modifier. Furthermore, the PMo12-Pt/rGO composite shows higher electrocatalytic activity, better tolerance towards CO and better stability than the conventional pure Pt catalyst.Catalysts2015-07-1753Article10.3390/catal5031275127512882073-43442015-07-17doi: 10.3390/catal5031275Xiaoying WangXiaofeng ZhangXiaolei HeAi MaLijuan LeShen Linhttp://www.mdpi.com/2073-4344/5/3/1221
This review selectively summarizes the latest developments in the Pd-based cataysts for low temperature proton exchange membrane fuel cells, especially in the application of formic acid oxidation, alcohol oxidation and oxygen reduction reaction. The advantages and shortcomings of the Pd-based catalysts for electrocatalysis are analyzed. The influence of the structure and morphology of the Pd materials on the performance of the Pd-based catalysts were described. Finally, the perspectives of future trends on Pd-based catalysts for different applications were considered.Catalysts2015-07-1553Review10.3390/catal5031221122112742073-43442015-07-15doi: 10.3390/catal5031221Hui MengDongrong ZengFangyan Xiehttp://www.mdpi.com/2073-4344/5/3/1211
It is of increasing interest and an important challenge to develop highly efficient less-expensive cathode catalysts for anion-exchange membrane fuel cells (AEMFCs). In this work, we have directly prepared a carbon-supported CoS nanocatalyst in a solvothermal route and investigated the effect of heat-treatment on electrocatalytic activity and long-term stability using rotating ring-disk electrode (RRDE). The results show that the heat-treatment below 400 °C under nitrogen atmosphere significantly enhanced the electrocatalytic performance of CoS catalyst as a function of annealed temperature in terms of the cathodic current density, the half-wave potential, the HO2− product and the number of electrons transferred. The CoS catalyst that annealed at 400 °C (CoS-400) has exhibited a promising performance with the half-wave potential of 0.71 V vs. RHE (the highest one for non-precious metal chalcogenides), the minimum HO2− product of 4.3% at 0.60 V vs. RHE and close to the 4-electron pathway during the oxygen reduction reaction in 0.1 M KOH. Also, the CoS-400 catalyst has comparable durability to the Pt/C catalyst.Catalysts2015-07-1553Article10.3390/catal5031211121112202073-43442015-07-15doi: 10.3390/catal5031211Haihong ZhongJingmin XiPinggui TangDianqing LiYongjun Fenghttp://www.mdpi.com/2073-4344/5/3/1202
Nitrogen and metal doped nanocarbons derived from polyaniline (PANI) have been widely explored as electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells. In this work, we report surfactant-template synthesis of PANI nanostructures and the ORR electrocatalysts derived from them. By using cationic surfactant such as the cetyl trimethyl ammonium bromide (CTAB) as the template and the negatively charged persulfate ions as the oxidative agent to stimulate the aniline polymerization in the micelles of CTAB, PANI with a unique 1-D semi-tubular structure can be obtained. The semi-tubular structure can be maintained even after high-temperature treatment at 900 °C, which yields materials exhibiting promising ORR activity.Catalysts2015-07-0753Communication10.3390/catal5031202120212102073-43442015-07-07doi: 10.3390/catal5031202Shiming ZhangShengli Chenhttp://www.mdpi.com/2073-4344/5/3/1193
First principles approach is used to examine geometric and electronic structure of the catalyst concept aimed to improve activity and utilization of precious Pt metal for oxygen reduction reaction in fuel cells. The Pt monolayers on Pd skin and Pd1-xCux inner core for various compositions x were examined by building the appropriate models starting from Pd-Cu solid solution. We provided a detailed description of changes in the descriptors of catalytic behavior, d-band energy and binding energies of reaction intermediates, giving an insight into the underlying mechanism of catalytic activity enhancement based on the first principles density functional theory (DFT) calculations. Structural properties of the Pd-Cu bimetallic were determined for bulk and surfaces, including the segregation profile of Cu under different environment on the surface.Catalysts2015-07-0653Article10.3390/catal5031193119312012073-43442015-07-06doi: 10.3390/catal5031193Amra PelesMinhua ShaoLesia Protsailohttp://www.mdpi.com/2073-4344/5/3/1167
In order to reduce the overall system cost, the development of inexpensive, high-performance and durable oxygen reduction reaction (ORR)N, Fe-codoped carbon-based (Fe/N/C) electrocatalysts to replace currently used Pt-based catalysts has become one of the major topics in research on fuel cells. This review paper lays the emphasis on introducing the progress made over the recent five years with a detailed discussion of recent work in the area of Fe/N/C electrocatalysts for ORR and the possible Fe-based active sites. Fe-based materials prepared by simple pyrolysis of transition metal salt, carbon support, and nitrogen-rich small molecule or polymeric compound are mainly reviewed due to their low cost, high performance, long stability and because they are the most promising for replacing currently used Pt-based catalysts in the progress of fuel cell commercialization. Additionally, Fe-base catalysts with small amount of Fe or new structure of Fe/Fe3C encased in carbon layers are presented to analyze the effect of loading and existence form of Fe on the ORR catalytic activity in Fe-base catalyst. The proposed catalytically Fe-centered active sites and reaction mechanisms from various authors are also discussed in detail, which may be useful for the rational design of high-performance, inexpensive, and practical Fe-base ORR catalysts in future development of fuel cells.Catalysts2015-07-0653Review10.3390/catal5031167116711922073-43442015-07-06doi: 10.3390/catal5031167Jing LiuErling LiMingbo RuanPing SongWeilin Xuhttp://www.mdpi.com/2073-4344/5/3/1152
Hydrodeoxygenation of guaiacol in the presence of 1-octanol was studied in a fixed-bed reactor under mild conditions (50–250 °C) over platinum particles supported on silica (Pt/SiO2) and a zeolite with framework type MFI at a Si/Al-ratio of 45 (Pt/H-MFI-90). The deoxygenation selectivity strongly depended on the support and the temperature. Both guaiacol and octanol were rapidly deoxygenated in the presence of hydrogen over Pt/H-MFI-90 at 250 °C to cyclohexane and octane, respectively. In contrast, Pt/SiO2 mostly showed hydrogenation, but hardly any deoxygenation activity. The acidic sites of the MFI-90 support lead to improved deoxygenation performance at the mild temperature conditions of this study. Significant conversions under reaction conditions applied already occurred at temperatures of 200 °C. However, during long-term stability tests, the Pt/H-MFI-90 catalyst deactivated after more than 30 h, probably due to carbon deposition, whereas Pt/SiO2 was more stable. The catalytic activity of the zeolite catalyst could only partly be regained by calcination in air, as some of the acidic sites were lost.Catalysts2015-07-0353Article10.3390/catal5031152115211662073-43442015-07-03doi: 10.3390/catal5031152Melanie HellingerSina BaierPeter MortensenWolfgang KleistAnker JensenJan-Dierk Grunwaldthttp://www.mdpi.com/2073-4344/5/3/1092
The treatment of volatile organic compounds (VOC) emissions is a necessity of today. The catalytic treatment has already proven to be environmentally and economically sound technology for the total oxidation of the VOCs. However, in certain cases, it may also become economical to utilize these emissions in some profitable way. Currently, the most common way to utilize the VOC emissions is their use in energy production. However, interesting possibilities are arising from the usage of VOCs in hydrogen and syngas production. Production of chemicals from VOC emissions is still mainly at the research stage. However, few commercial examples exist. This review will summarize the commercially existing VOC utilization possibilities, present the utilization applications that are in the research stage and introduce some novel ideas related to the catalytic utilization possibilities of the VOC emissions. In general, there exist a vast number of possibilities for VOC utilization via different catalytic processes, which creates also a good research potential for the future.Catalysts2015-07-0153Review10.3390/catal5031092109211512073-43442015-07-01doi: 10.3390/catal5031092Satu OjalaNiina KoivikkoTiina LaitinenAnass MouamminePrem SeelamSaid LaassiriKaisu AinassaariRachid BrahmiRiitta Keiskihttp://www.mdpi.com/2073-4344/5/3/1079
The carbon supported iron selenide catalysts (FeSe2/C) were prepared with various selenium to iron ratios (Se/Fe), namely, Se/Fe = 2.0, 2.5, 3.0, 3.5 and 4.0, through facile microwave route by using ferrous oxalate (FeC2O4·2H2O) and selenium dioxide (SeO2) as precursors. Accordingly, effects of Se/Fe ratio on the crystal structure, crystallite size, microstructure, surface composition and electrocatalytic activity for oxygen reduction reaction (ORR) of FeSe2/C in an alkaline medium were systematically investigated. The results revealed that all the FeSe2/C catalysts obtained with the Se/Fe ratios of 2.0–4.0 exhibited almost pure orthogonal FeSe2 structure with the estimated mean crystallite sizes of 32.9–36.2 nm. The electrocatalytic activities in potassium hydroxide solutions were higher than those in perchloric acid solutions, and two peak potentials or two plateaus responded to ORR were observed from cyclic voltammograms and polarization curves, respectively. The ORR potentials of 0.781–0.814 V with the electron transfer numbers of 3.3–3.9 at 0.3 V could be achieved as the Se/Fe ratios varied from 2.0 to 4.0. The Fe and Se were presented at the surface of FeSe2/C upon further reduction on FeSe2. The Se/Fe ratios slightly influenced the degree of graphitization in carbon support and the amount of active sites for ORR. Catalysts2015-06-3053Article10.3390/catal5031079107910912073-43442015-06-30doi: 10.3390/catal5031079Qiaoling ZhengXuan ChengHengyi Lihttp://www.mdpi.com/2073-4344/5/3/1068
A series of PdW alloy supported on Vulcan XC-72 Carbon (PdW/C) with total 20 wt. % as electrocatalyst are prepared for ethanol oxidation by an ethylene glycol assisted method. Transmission electron microscopy (TEM) characterization shows that PdW nanoparticles with an average size of 3.6 nm are well dispersed on the surface of Vulcan XC-72 Carbon. It is found that the catalytic activity and stability of the PdW/C catalysts are strongly dependent on Pd/W ratios, an optimal Pd/W composition at 1/1 ratio revealed the highest catalytic activity toward ethanol oxidation, which is much better than commercial Pd/C catalysts.Catalysts2015-06-2953Article10.3390/catal5031068106810782073-43442015-06-29doi: 10.3390/catal5031068Qi LiuMingshuang LiuQiaoxia LiQunjie Xuhttp://www.mdpi.com/2073-4344/5/3/1046
Over the past decade; a significant amount of research has been performed on novel carbon supports for use in proton exchange membrane fuel cells (PEMFCs). Specifically, carbon nanotubes, ordered mesoporous carbon, and colloid imprinted carbons have shown great promise for improving the activity and/or stability of Pt-based nanoparticle catalysts. In this work, a brief overview of these materials is given, followed by an in-depth discussion of our recent work highlighting the importance of carbon wall thickness when designing novel carbon supports for PEMFC applications. Four colloid imprinted carbons (CICs) were synthesized using a silica colloid imprinting method, with the resulting CICs having pores of 15 (CIC-15), 26 (CIC-26), 50 (CIC-50) and 80 (CIC-80) nm. These four CICs were loaded with 10 wt. % Pt and then evaluated as oxygen reduction (ORR) catalysts for use in proton exchange membrane fuel cells. To gain insight into the poorer performance of Pt/CIC-26 vs. the other three Pt/CICs, TEM tomography was performed, indicating that CIC-26 had much thinner walls (0–3 nm) than the other CICs and resulting in a higher resistance (leading to distributed potentials) through the catalyst layer during operation. This explanation for the poorer performance of Pt/CIC-26 was supported by theoretical calculations, suggesting that the internal wall thickness of these nanoporous CICs is critical to the future design of porous carbon supports.Catalysts2015-06-2953Article10.3390/catal5031046104610672073-43442015-06-29doi: 10.3390/catal5031046Dustin BanhamFangxia FengTobias FürstenhauptKatie PeiSiyu YeViola Birsshttp://www.mdpi.com/2073-4344/5/3/1034
Nitrogen-doped ordered mesoporous carbon was synthesized by using polyaniline as the carbon source and SBA-15 as the template. The microstructure, composition and electrochemical behavior were extensively investigated by the nitrogen sorption isotherm, X-ray photoelectron spectroscopy, cyclic voltammetry and rotating ring-disk electrode. It is found that the pyrolysis temperature yielded a considerable effect on the pore structure, elemental composition and chemical configuration. The pyrolysis temperature from 800 to 1100 °C yielded a volcano-shape relationship with both the specific surface area and the content of the nitrogen-activated carbon. Electrochemical tests showed that the electrocatalytic activity followed a similar volcano-shape relationship, and the carbon catalyst synthesized at 1000 °C yielded the best performance. The post-treatment in NH3 was found to further increase the specific surface area and to enhance the nitrogen doping, especially the edge-type nitrogen, which favored the oxygen reduction reaction in both acid and alkaline media. The above findings shed light on electrocatalysis and offer more strategies for the controllable synthesis of the doped carbon catalyst.Catalysts2015-06-2653Article10.3390/catal5031034103410452073-43442015-06-26doi: 10.3390/catal5031034Kai WanZhi-Peng YuZhen-Xing Lianghttp://www.mdpi.com/2073-4344/5/3/1016
In this work, sodium citrate (SC) was used as an additive to control the particle size and dispersion of Pt-Mn alloy nanoparticles deposited on a carbon support. SC was chosen, since it was the only additive tested that did not prevent Mn from co-depositing with Pt. The influence of solution pH during deposition and post-deposition heat treatment on the physical and electrochemical properties of the Pt-Mn alloy was examined. It was determined that careful control over pH is required, since above a pH of four, metal deposition was suppressed. Below pH 4, the presence of sodium citrate reduced the particle size and improved the particle dispersion. This also resulted in larger electrochemically-active surface areas and greater activity towards the ethanol oxidation reaction (EOR). Heat treatment of catalysts prepared using the SC additive led to a significant enhancement in EOR activity, eclipsing the highest activity of our best Pt-Mn/C prepared in the absence of SC. XRD studies verified the formation of the Pt-Mn intermetallic phase upon heat treatment. Furthermore, transmission electron microscopy studies revealed that catalysts prepared using the SC additive were more resistant to particle size growth during heat treatment.Catalysts2015-06-2553Article10.3390/catal5031016101610332073-43442015-06-25doi: 10.3390/catal5031016Mohammadreza GhavidelE. Eastonhttp://www.mdpi.com/2073-4344/5/3/1003
We have prepared a multi-component nanoporous PtRuCuW (np-PtRuCuW) electrocatalyst via a combined chemical dealloying and mechanical alloying process. The X-ray diffraction (XRD), transmission electron microscopy (TEM) and electrochemical measurements have been applied to characterize the microstructure and electrocatalytic activities of the np-PtRuCuW. The np-PtRuCuW catalyst has a unique three-dimensional bi-continuous ligament structure and the length scale is 2.0 ± 0.3 nm. The np-PtRuCuW catalyst shows a relatively high level of activity normalized to mass (467.1 mA mgPt−1) and electrochemically active surface area (1.8 mA cm−2) compared to the state-of-the-art commercial PtC and PtRu catalyst at anode. Although the CO stripping peak of np-PtRuCuW 0.47 V (vs. saturated calomel electrode, SCE) is more positive than PtRu, there is a 200 mV negative shift compared to PtC (0.67 V vs. SCE). In addition, the half-wave potential and specific activity towards oxygen reduction of np-PtRuCuW are 0.877 V (vs. reversible hydrogen electrode, RHE) and 0.26 mA cm−2, indicating a great enhancement towards oxygen reduction than the commercial PtC.Catalysts2015-06-2453Article10.3390/catal5031003100310152073-43442015-06-24doi: 10.3390/catal5031003Xiaoting ChenHao WangYing WangQingguo BaiYulai GaoZhonghua Zhanghttp://www.mdpi.com/2073-4344/5/2/992
Large scale syntheses of uniform metal nanoparticles with hollow porous structure have attracted much attention owning to their high surface area, abundant active sites and relatively efficient catalytic activity. Herein, we report a general method to synthesize hollow porous Pd nanospheres (Pd HPNSs) by templating sacrificial SiO2 nanoparticles with the assistance of polyallylamine hydrochloride (PAH) through layer-by-layer self-assembly. The chemically inert PAH is acting as an efficient stabilizer and complex agent to control the synthesis of Pd HPNSs, probably accounting for its long aliphatic alkyl chains, excellent coordination capability and good hydrophilic property. The physicochemical properties of Pd HPNSs are thoroughly characterized by various techniques, such as transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy. The growth mechanism of Pd HPNSs is studied based on the analysis of diverse experimental observations. The as-prepared Pd HPNSs exhibit clearly enhanced electrocatalytic activity and durability for the formic oxidation reaction (FAOR) in acid medium compared with commercial Pd black.Catalysts2015-06-2352Article10.3390/catal502099299210022073-43442015-06-23doi: 10.3390/catal5020992Xiaoyu QiuHanyue ZhangYuxuan DaiFengqi ZhangPeishan WuPin WuYawen Tanghttp://www.mdpi.com/2073-4344/5/2/981
A P, N dual doped reduced graphene oxide (PN-rGO) catalyst with high surface area (376.20 m2·g−1), relatively high P-doping level (1.02 at. %) and a trace amount of N (0.35 at. %) was successfully prepared using a one-step method by directly pyrolyzing a homogenous mixture of graphite oxide (GO) and diammonium hydrogen phosphate ((NH4)2HPO4) in an argon atmosphere, during which the thermal expansion, deoxidization of GO and P, N co-doping were realized simultaneously. The catalyst exhibited enhanced catalytic performances for oxygen reduction reaction (ORR) via a dominated four-electron reduction pathway, as well as superior long-term stability, better tolerance to methanol crossover than that of commercial Pt/C catalyst in an alkaline solution.Catalysts2015-06-2352Article10.3390/catal50209819819912073-43442015-06-23doi: 10.3390/catal5020981Xiaochang QiaoShijun LiaoChenghang YouRong Chenhttp://www.mdpi.com/2073-4344/5/2/966
We have prepared Pt nanoparticles supported on titanium carbide (TiC) (Pt/TiC) as an alternative cathode catalyst with high durability at high potentials for polymer electrolyte fuel cells. The Pt/TiC catalysts with and without heat treatment were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Hemispherical Pt nanocrystals were found to be dispersed uniformly on the TiC support after heat treatment at 600 °C in 1% H2/N2 (Pt/TiC-600 °C). The electrochemical properties (cyclic voltammetry, electrochemically active area (ECA), and oxygen reduction reaction (ORR) activity) of Pt/TiC-600 °C and a commercial Pt/carbon black (c-Pt/CB) were evaluated by the rotating disk electrode (RDE) technique in 0.1 M HClO4 solution at 25 °C. It was found that the kinetically controlled mass activity for the ORR on Pt/TiC-600 °C at 0.85 V (507 A g−1) was comparable to that of c-Pt/CB (527 A g−1). Moreover, the durability of Pt/TiC-600 °C examined by a standard potential step protocol (E = 0.9 V↔1.3 V vs. RHE, holding 30 s at each E) was much higher than that for c-Pt/CB.Catalysts2015-06-2352Article10.3390/catal50209669669802073-43442015-06-23doi: 10.3390/catal5020966Morio ChiwataKatsuyoshi KakinumaMitsuru WakisakaMakoto UchidaShigehito DekiMasahiro WatanabeHiroyuki Uchidahttp://www.mdpi.com/2073-4344/5/2/955
Finding inexpensive alternatives to platinum group metals (PGMs) is essential for reducing the cost of proton exchange membrane fuel cells (PEMFCs). Numerous materials have been investigated as potential replacements of Pt, of which the transition metal and nitrogen-doped carbon composites (TM/Nx/C) prepared from iron doped zeolitic imidazolate frameworks (ZIFs) are among the most active ones in catalyzing the oxygen reduction reaction based on recent studies. In this report, we demonstrate that the catalytic activity of ZIF-based TM/Nx/C composites can be substantially improved through optimization of synthesis and post-treatment processing conditions. Ultimately, oxygen reduction reaction (ORR) electrocatalytic activity must be demonstrated in membrane-electrode assemblies (MEAs) of fuel cells. The process of preparing MEAs using ZIF-based non-PGM electrocatalysts involves many additional factors which may influence the overall catalytic activity at the fuel cell level. Evaluation of parameters such as catalyst loading and perfluorosulfonic acid ionomer to catalyst ratio were optimized. Our overall efforts to optimize both the catalyst and MEA construction process have yielded impressive ORR activity when tested in a fuel cell system.Catalysts2015-06-1152Article10.3390/catal50209559559652073-43442015-06-11doi: 10.3390/catal5020955Heather BarkholtzLina ChongZachary KaiserTao XuDi-Jia Liuhttp://www.mdpi.com/2073-4344/5/2/949
Catalyst deactivation, the loss over time of catalytic activity and/or selectivity, is a problem of great and continuing concern in the practice of industrial catalytic processes. Costs to industry for catalyst replacement and process shutdown total tens of billions of dollars per year. [...]Catalysts2015-06-1152Editorial10.3390/catal50209499499542073-43442015-06-11doi: 10.3390/catal5020949Calvin BartholomewMorris Argylehttp://www.mdpi.com/2073-4344/5/2/926
The initial performance and decay trends of polymer electrolyte membrane fuel cells (PEMFC) cathodes with Pt3Co catalysts of three mean particle sizes (4.9 nm, 8.1 nm, and 14.8 nm) with identical Pt loadings are compared. Even though the cathode based on 4.9 nm catalyst exhibited the highest initial electrochemical surface area (ECA) and mass activity, the cathode based on 8.1 nm catalyst showed better initial performance at high currents. Owing to the low mass activity of the large particles, the initial performance of the 14.8 nm Pt3Co-based electrode was the lowest. The performance decay rate of the electrodes with the smallest Pt3Co particle size was the highest and that of the largest Pt3Co particle size was lowest. Interestingly, with increasing number of decay cycles (0.6 to 1.0 V, 50 mV/s), the relative improvement in performance of the cathode based on 8.1 nm Pt3Co over the 4.9 nm Pt3Co increased, owing to better stability of the 8.1 nm catalyst. The electron microprobe analysis (EMPA) of the decayed membrane-electrode assembly (MEA) showed that the amount of Co in the membrane was lower for the larger particles, and the platinum loss into the membrane also decreased with increasing particle size. This suggests that the higher initial performance at high currents with 8.1 nm Pt3Co could be due to lower contamination of the ionomer in the electrode. Furthermore, lower loss of Co from the catalyst with increased particle size could be one of the factors contributing to the stability of ECA and mass activity of electrodes with larger cathode catalyst particles. To delineate the impact of particle size and alloy effects, these results are compared with prior work from our research group on size effects of pure platinum catalysts. The impact of PEMFC operating conditions, including upper potential, relative humidity, and temperature on the alloy catalyst decay trends, along with the EMPA analysis of the decayed MEAs, are reported.Catalysts2015-05-2952Article10.3390/catal50209269269482073-43442015-05-29doi: 10.3390/catal5020926Mallika GummallaSarah BallDavid ConditSomaye RasouliKang YuPaulo FerreiraDeborah MyersZhiwei Yanghttp://www.mdpi.com/2073-4344/5/2/905
Co-Mn-Mg-Al oxides were synthesized using auto-combustion and co-precipitation techniques. Constant ratios were maintained with (Co + Mn + Mg)/Al equal to 3.0, (Co + Mn)/Mg equal to 1.0 and Co/Mn equal to 0.5. The chemical and structural composition, redox properties, oxygen storage capacity and oxygen mobility were analyzed using X-ray fluorescence (XRF), X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), temperature-programmed reduction of hydrogen (H2-TPR), oxygen storage capacity (OSC), oxygen storage complete capacity (OSCC) and isotopic exchange, respectively. The catalytic behavior of the oxides was evaluated in the total oxidation of a mixture of 250 ppm toluene and 250 ppm 2-propanol. The synthesis methodology affected the crystallite size, redox properties, OSC and oxide oxygen mobility, which determined the catalytic behavior. The co-precipitation method got the most active oxide in the oxidation of the volatile organic compound (VOC) mixture because of the improved mobility of oxygen and ability to favor redox processes in the material structure.Catalysts2015-05-2952Article10.3390/catal50209059059252073-43442015-05-29doi: 10.3390/catal5020905María CastañoRafael MolinaSonia Morenohttp://www.mdpi.com/2073-4344/5/2/871
An experimental and kinetic modeling study on the Ni-catalyzed conversion of methane under oxidative and reforming conditions is presented. The numerical model is based on a surface reaction mechanism consisting of 52 elementary-step like reactions with 14 surface and six gas-phase species. Reactions for the conversion of methane with oxygen, steam, and CO2 as well as methanation, water-gas shift reaction and carbon formation via Boudouard reaction are included. The mechanism is implemented in a one-dimensional flow field description of a fixed bed reactor. The model is evaluated by comparison of numerical simulations with data derived from isothermal experiments in a flow reactor over a powdered nickel-based catalyst using varying inlet gas compositions and operating temperatures. Furthermore, the influence of hydrogen and water as co-feed on methane dry reforming with CO2 is also investigated.Catalysts2015-05-2952Article10.3390/catal50208718719042073-43442015-05-29doi: 10.3390/catal5020871Karla DelgadoLubow MaierSteffen TischerAlexander ZellnerHenning StotzOlaf Deutschmannhttp://www.mdpi.com/2073-4344/5/2/868
Heterogeneous metal catalysts rather than homogeneous ones are recommended for industrial applications after considering their performance in activity, separation, and recycling [1]. The recycling of metal catalysts is important from economic and environmental points of view. When supported and bulk metal catalysts are used in liquid-phase organic reactions, there is a possibility that active metal species are leaching away into the liquid phases [2,3]. The metal leaching would make it difficult for the catalysts to maintain their desired initial performance for repeated batch reactions and during continuous ones. The metal leaching would also cause some undesired contamination of products by the metal species dissolved in the reaction mixture, and the separation of the metal contaminants would be required to purify the products. Therefore, various novel methods have been proposed so far to immobilize/stabilize the active metal species and to separate/collect/reuse the dissolved metal species [4]. In addition, knowledge on the heterogeneous and homogeneous natures of organic reactions using heterogeneous catalysts is important to discuss their reaction mechanisms and catalytically working active species. [...]Catalysts2015-05-2752Editorial10.3390/catal50208688688702073-43442015-05-27doi: 10.3390/catal5020868Masahiko AraiFengyu Zhaohttp://www.mdpi.com/2073-4344/5/2/851
Co6Al2HT hydrotalcite-like compounds were synthesized by three different methods: co-precipitation, microwaves-assisted and ultrasound-assisted methods. The mixed oxides obtained after calcination were studied by several techniques: XRD, TEM, H2-TPR and XPS. They were also tested as catalysts in the reaction of total oxidation of toluene. The physico-chemical studies revealed a modification of the structural characteristics (surface area, morphology) as well as of the reducibility of the formed mixed oxides. The solid prepared by microwaves-assisted synthesis was the most active. Furthermore, a relationship between the ratio of Co2+ on the surface, the reducibility of the Co-Al mixed oxide and the T50 in toluene oxidation was demonstrated. This suggests a Mars Van Krevelen mechanism for toluene total oxidation on these catalysts.Catalysts2015-05-1552Article10.3390/catal50208518518672073-43442015-05-15doi: 10.3390/catal5020851Eric GentyJulien BrunetChristophe PoupinSandra CasaleSylvie CapellePascale MassianiStéphane SiffertRenaud Cousinhttp://www.mdpi.com/2073-4344/5/2/838
We investigated the reaction mechanism of a metal-unloaded zeolite-plasma hybrid system for decomposition of toluene at room temperature. Short-lived radicals and fast electrons did not contribute substantially to the reaction mechanism of toluene decomposition in the zeolite-plasma hybrid system. The main factor enhancing the reaction mechanism was gas-phase O3 directly reacting with toluene adsorbed onto the zeolite (the Eley-Rideal mechanism). CO2 selectivity was not improved by using H-Y zeolite due to its low ability to retain the active oxygen species formed by O3. The gas-phase O3 reacted with adsorbed toluene to form a ring cleavage intermediate that was slowly converted into formic acid. The decomposition rate of formic acid was much lower than that of toluene on the H-Y surface.Catalysts2015-05-1152Article10.3390/catal50208388388502073-43442015-05-11doi: 10.3390/catal5020838Yoshiyuki TeramotoHyun-Ha KimNobuaki NegishiAtsushi Ogatahttp://www.mdpi.com/2073-4344/5/2/815
The synthesis of core-shell Pt(Cu) and Pt-Ru(Cu) electrocatalysts allows for a reduction in the amount of precious metal and, as was previously shown, a better CO oxidation performance can be achieved when compared to the nanoparticulated Pt and Pt-Ru ones. In this paper, the carbon black used as the support was previously submitted to electrochemical oxidation and characterized by XPS. The new catalysts thus prepared were characterized by HRTEM, FFT, EDX, and electrochemical techniques. Cu nanoparticles were generated by electrodeposition and were further transformed into Pt(Cu) and Pt-Ru(Cu) core-shell nanoparticles by successive galvanic exchange with Pt and spontaneous deposition of Ru species, the smallest ones being 3.3 nm in mean size. The onset potential for CO oxidation was as good as that obtained for the untreated carbon, with CO stripping peak potentials about 0.1 and 0.2 V more negative than those corresponding to Pt/C and Ru-decorated Pt/C, respectively. Carbon oxidation yielded an additional improvement in the catalyst performance, because the ECSA values for hydrogen adsorption/desorption were much higher than those obtained for the non-oxidized carbon. This suggested a higher accessibility of the Pt sites in spite of having the same nanoparticle structure and mean size.Catalysts2015-04-3052Article10.3390/catal50208158158372073-43442015-04-30doi: 10.3390/catal5020815Griselda Caballero-ManriqueEnric BrillasFrancesc CentellasJosé GarridoRosa RodríguezPere-Lluís Cabothttp://www.mdpi.com/2073-4344/5/2/800
The removal of dilute diethylether (DEE, concentration: 150 ppm) from an air stream (flow rate: 1.0 L min−1) using non-thermal plasma combined with different cordierite-supported catalysts, including Mn, Fe, and mixed Mn-Fe oxides, was investigated. The experimental results showed that the decomposition of DEE occurred in a one-stage reactor without the positive synergy of plasma and supported catalysts, by which ca. 96% of DEE was removed at a specific input energy (SIE) of ca. 600 J L−1, except when the mixed Mn-Fe/cordierite was used. Among the catalysts that were examined, Mn-Fe/cordierite, the catalyst that was the most efficient at decomposing ozone was found to negatively affect the decomposition of DEE in the one-stage reactor. However, when it was utilized as a catalyst in the post-plasma stage of a two-part hybrid reactor, in which Mn/cordierite was directly exposed to the plasma, the reactor performance in terms of DEE decomposition efficiency was improved by more than 10% at low values of SIE compared to the efficiency that was achieved without Mn-Fe/cordierite. The ozone that was formed during the plasma stage and its subsequent catalytic dissociation during the post-plasma stage to produce atomic oxygen therefore played important roles in the removal of DEE.Catalysts2015-04-2952Article10.3390/catal50208008008142073-43442015-04-29doi: 10.3390/catal5020800Quang TrinhYoung Mokhttp://www.mdpi.com/2073-4344/5/2/774
The catalytic combustion of volatile organic compounds (VOCs) is one of the most important techniques to remove these pollutants from the air stream, but it should be carried out at the lowest possible temperature, saving energy and avoiding the simultaneous formation of nitrogen oxides (NOx). Under these experimental conditions, the chemisorption of water generated from VOCs combustion may inhibit hydrophilic catalysts. Nowadays, a wide variety of carbon materials is available to be used in catalysis. The behavior of these hydrophobic materials in the development of highly active and selective combustion catalysts is analyzed in this manuscript. The support characteristics (porosity, hydrophobicity, structure, surface chemistry, etc.) and the active phase nature (noble metals: Pt, Pd) and dispersion were analyzed by several techniques and the results correlated with the dual adsorptive and/or catalytic performance of the corresponding catalysts. The coupling of highly active phases and carbon materials (activated carbons, honeycomb coated monoliths, carbon aerogels, etc.) with tuneable physicochemical properties leads to the complete abatement of benzene, toluene and xylenes (BTX) from dilute air streams, being selectively oxidized to CO2 at low temperatures.Catalysts2015-04-2752Review10.3390/catal50207747747992073-43442015-04-27doi: 10.3390/catal5020774Sergio Morales-TorresFrancisco Carrasco-MarínAgustín Pérez-CadenasFrancisco Maldonado-Hódarhttp://www.mdpi.com/2073-4344/5/2/759
Ni-based catalysts as replacement for noble metal catalysts are of particular interest in the catalytic conversion of biomass due to their cheap and satisfactory catalytic activity. The Ni/SiO2 catalyst has been studied for the hydrogenolysis of glycerol, and doping with phosphorus (P) found to improve the catalytic performance significantly because of the formation of Ni2P alloys. However, in the present work we disclose a different catalytic phenomenon for the P-doped Ni/Al2O3 catalyst. We found that doping with P has a significant effect on the state of the active Ni species, and thus improves the selectivity to 1,2-propanediol (1,2-PDO) significantly in the hydrogenolysis of glycerol, although Ni-P alloys were not observed in our catalytic system. The structure and selectivity correlations were determined from the experimental data, combining the results of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H2-TPR) and ammonia temperature-programmed desorption (NH3-TPD). The presence of NiO species, formed from P-doped Ni/Al2O3 catalyst, was shown to benefit the formation of 1,2-PDO. This was supported by the results of the Ni/Al2O3 catalyst containing NiO species with incomplete reduction. Furthermore, the role the NiO species played in the reaction and the potential reaction mechanism over the P-doped Ni/Al2O3 catalyst is discussed. The new findings in the present work open a new vision for Ni catalysis and will benefit researchers in designing Ni-based catalysts.Catalysts2015-04-2352Article10.3390/catal50207597597732073-43442015-04-23doi: 10.3390/catal5020759Xiaoru LiHaiyang ChengGuanfeng LiangLimin HeWeiwei LinYancun YuFengyu Zhaohttp://www.mdpi.com/2073-4344/5/2/747
In this paper, a catalyst of hollow PdCu alloy nanocubes supported on nitrogen-doped graphene support (H-PdCu/ppy-NG) is successfully synthesized using a simple one-pot template-free method. Two other catalyst materials such as solid PdCu alloy particles supported on this same nitrogen-doped graphene support (PdCu/ppy-NG) and hollow PdCu alloy nanocubes supported on the reduced graphene oxide support (H-PdCu/RGO) are also prepared using the similar synthesis conditions for comparison. It is found that, among these three catalyst materials, H-PdCu/ppy-NG gives the highest electrochemical active area and both the most uniformity and dispersibility of H-PdCu particles. Electrochemical tests show that the H-PdCu/ppy-NG catalyst can give the best electrocatalytic activity and stability towards the ethanol electrooxidation when compared to other two catalysts. Therefore, H-PdCu/ppy-NG should be a promising catalyst candidate for anodic ethanol oxidation in direct ethanol fuel cells. Catalysts2015-04-2352Article10.3390/catal50207477477582073-43442015-04-23doi: 10.3390/catal5020747Zhengyu BaiRumeng HuangLu NiuQing ZhangLin YangJiujun Zhanghttp://www.mdpi.com/2073-4344/5/2/718
Energy consumption is an important concern for the removal of volatile organic compounds (VOCs) from waste air with non-thermal plasma (NTP). Although the combination of NTP with heterogeneous catalysis has shown to reduce the formation of unwanted by-products and improve the energy efficiency of the process, further optimization of these hybrid systems is still necessary to evolve to a competitive air purification technology. A newly developed innovative technique, i.e., the cyclic operation of VOC adsorption and NTP-assisted regeneration has attracted growing interest of researchers due to the optimized energy consumption and cost-effectiveness. This paper reviews this new technique for the abatement of VOCs as well as for regeneration of adsorbents. In the first part, a comparison of the energy consumption between sequential and continuous treatment is given. Next, studies dealing with adsorption followed by NTP oxidation are reviewed. Particular attention is paid to the adsorption mechanisms and the regeneration of catalysts with in-plasma and post-plasma processes. Finally, the influence of critical process parameters on the adsorption and regeneration steps is summarized.Catalysts2015-04-2352Review10.3390/catal50207187187462073-43442015-04-23doi: 10.3390/catal5020718Sharmin SultanaArne VandenbrouckeChristophe LeysNathalie De GeyterRino Morenthttp://www.mdpi.com/2073-4344/5/2/703
In this study, multifunctional catalysts were developed by adding *BEA or MFI zeolite with high Si/Al ratio to a residual fluidized catalytic cracking (RFCC) catalyst and tested in the catalytic cracking of n-hexadecane, which is a heavy crude oil model compound, for the purpose of increasing the octane value of produced gasoline under the strong hydrogen transfer activity of the RFCC catalyst. Reaction products analysis revealed that the addition of *BEA zeolite to the RFCC catalyst increased the yields of olefins and multi-branched paraffins, which resulted in improvement of the octane value without sacrificing gasoline yield. On the contrary, the addition of MFI zeolite decreased the gasoline yield because it cracks the gasoline range olefins into LPG range olefins. In general, it is difficult to increase the yield of multi-branched molecules because the multi-branched molecule is more easily cracked than linear molecules. Our results suggest the possibility for the selective acceleration of isomerization reaction by the addition of less acidic *BEA zeolite to the RFCC catalyst.Catalysts2015-04-2252Article10.3390/catal50207037037172073-43442015-04-22doi: 10.3390/catal5020703Iori ShimadaRyoichi ImaiYoshinori HayasakiHiroshi FukunagaNobuhide TakahashiToru Takatsukahttp://www.mdpi.com/2073-4344/5/2/690
A range of catalysts comprising of platinum supported on silica, prepared by an impregnation method, have been studied for the total oxidation of naphthalene, which is a representative Polycyclic Aromatic Hydrocarbon. The influence of platinum loading and calcination temperature on oxidation activity was evaluated. Increasing the platinum loading up to 2.5 wt.% increased the catalyst activity, whilst a 5.0 wt.% catalyst was slightly less active. The catalyst containing the optimum 2.5 wt.% loading was most active after calcination in air at 550 °C. Characterisation by carbon monoxide chemisorption and X-ray photoelectron spectroscopy showed that low platinum dispersion to form large platinum particles, in combination with platinum in metallic and oxidised states was important for high catalyst activity. Catalyst performance improved after initial use in repeat cycles, whilst there was slight deactivation after prolonged time-on-stream.Catalysts2015-04-2252Article10.3390/catal50206906907022073-43442015-04-22doi: 10.3390/catal5020690David SellickDavid MorganStuart Taylorhttp://www.mdpi.com/2073-4344/5/2/671
In the following work, the catalytic behavior of Pd catalysts prepared using different oxides as support (Al2O3, CeO2 and TiO2) in the catalytic combustion of propene, in low concentration in excess of oxygen, to mimic the conditions of catalytic decomposition of a volatile organic compound of hydrocarbon-type is reported. In addition, the influence of different promoters (Ce, Ti, Fe and Mn) when added to a Pd/Al2O3 catalyst was analyzed. Catalysts were prepared by the impregnation method and were characterized by ICP-OES, N2 adsorption, temperature-programmed reduction, temperature-programmed oxidation, X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy. Catalyst prepared using CeO2 as the support was less easily reducible, due to the stabilization effect of CeO2 over the palladium oxides. Small PdO particles and, therefore, high Pd dispersion were observed for all of the catalysts, as confirmed by XRD and TEM. The addition of Ce to the Pd/Al2O3 catalysts increased the metal-support interaction and the formation of highly-dispersed Pd species. The addition of Ce and Fe improved the catalytic behavior of the Pd/Al2O3 catalyst; however, the addition of Mn and Ti decreased the catalytic activity in the propene oxidation. Pd/TiO2 showed the highest catalytic activity, probably due to the high capacity of this catalyst to reoxidize Pd into PdO, as has been found in the temperature-programmed oxidation (TPO) experiments.Catalysts2015-04-2252Article10.3390/catal50206716716892073-43442015-04-22doi: 10.3390/catal5020671Sonia GilJesus Garcia-VargasLeonarda LiottaGiuseppe PantaleoMohamed OusmaneLaurence RetailleauAnne Giroir-Fendlerhttp://www.mdpi.com/2073-4344/5/2/653
Avoiding strong chemical bonding, as indicated by lower heat of adsorption value, is among the selection criteria for Volatile Organic Compounds adsorbents. In this work, we highlight a non-calorimetric approach to estimating the energy of adsorption and desorption based on measurement of involved amounts, under dynamic conditions, with gaseous Fourier Transform Infrared spectroscopy. The collected data were used for obtaining adsorption heat values through the application of three different methods, namely, isosteric, temperature programmed desorption (TPD), and temperature-programmed adsorption equilibrium (TPAE). The resulting values were compared and discussed with the scope of turning determination of the heat of adsorption with non-calorimetric methods into a relevant decision making tool for designing cost-effective and safe operating of adsorption facilities.Catalysts2015-04-2252Article10.3390/catal50206536536702073-43442015-04-22doi: 10.3390/catal5020653Abdelhamid KorrirAchraf KasmiMhamed AssebbanAhmed SouiknySoukaina HaffaneOuafae AchakTarik Chafikhttp://www.mdpi.com/2073-4344/5/2/634
The solid–liquid phase C-alkylation of active methylene containing compounds with C=O or P=O functions under phase transfer catalysis or microwave conditions has been summarized in this minireview. The mono- and dialkylation of the methylene containing derivatives was investigated under microwave (MW) conditions. It was found that in many cases, there was no need to use phase transfer catalyst under MW conditions. Moreover, most of the reactions were carried out without any solvent. These results mean a serious green chemical advantage.Catalysts2015-04-2152Review10.3390/catal50206346346522073-43442015-04-21doi: 10.3390/catal5020634Alajos GrünErika BálintGyörgy Keglevichhttp://www.mdpi.com/2073-4344/5/2/606
Conventional supported Pt catalysts have often been prepared by loading Pt onto commercial supports, such as SiO2, TiO2, Al2O3, and carbon. These catalysts usually have simple metal-support (i.e., Pt-SiO2) interfaces. To tune the catalytic performance of supported Pt catalysts, it is desirable to modify the metal-support interfaces by incorporating an oxide additive into the catalyst formula. Here we prepared three series of metal oxide-modified Pt catalysts (i.e., Pt/MOx/SiO2, Pt/MOx/TiO2, and Pt/MOx/Al2O3, where M = Al, Fe, Co, Cu, Zn, Ba, La) for CO oxidation. Among them, Pt/CoOx/SiO2, Pt/CoOx/TiO2, and Pt/CoOx/Al2O3 showed the highest catalytic activities. Relevant samples were characterized by N2 adsorption-desorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), H2 temperature-programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), CO temperature-programmed desorption (CO-TPD), O2 temperature-programmed desorption (O2-TPD), and CO2 temperature-programmed desorption (CO2-TPD).Catalysts2015-04-2052Article10.3390/catal50206066066332073-43442015-04-20doi: 10.3390/catal5020606Hongmei QinXiaoshuang QianTao MengYi LinZhen Mahttp://www.mdpi.com/2073-4344/5/2/595
Heavily substituted cyclopropane esters were prepared in high yields, complete diastereoselection and average (up to 58%) enantioselectivity. The reaction described herein entailed reacting 4-nitro-5-bromostyrylisoxazoles, a class of powerful Michael acceptors with malonate esters under the catalysis of 5 mol% of a chincona derived phase-transfer catalyst.Catalysts2015-04-1352Article10.3390/catal50205955956052073-43442015-04-13doi: 10.3390/catal5020595Linda PirasMaria MocciaMauro CortigianiMauro Adamohttp://www.mdpi.com/2073-4344/5/2/561
Effects of H2O on the activity and deactivation of Pd catalysts used for the oxidation of unburned CH4 present in the exhaust gas of natural-gas vehicles (NGVs) are reviewed. CH4 oxidation in a catalytic converter is limited by low exhaust gas temperatures (500–550 °C) and low concentrations of CH4 (400–1500 ppmv) that must be reacted in the presence of large quantities of H2O (10–15%) and CO2 (15%), under transient exhaust gas flows, temperatures, and compositions. Although Pd catalysts have the highest known activity for CH4 oxidation, water-induced sintering and reaction inhibition by H2O deactivate these catalysts. Recent studies have shown the reversible inhibition by H2O adsorption causes a significant drop in catalyst activity at lower reaction temperatures (below 450 °C), but its effect decreases (water adsorption becomes more reversible) with increasing reaction temperature. Thus above 500 °C H2O inhibition is negligible, while Pd sintering and occlusion by support species become more important. H2O inhibition is postulated to occur by either formation of relatively stable Pd(OH)2 and/or partial blocking by OH groups of the O exchange between the support and Pd active sites thereby suppressing catalytic activity. Evidence from FTIR and isotopic labeling favors the latter route. Pd catalyst design, including incorporation of a second noble metal (Rh or Pt) and supports high O mobility (e.g., CeO2) are known to improve catalyst activity and stability. Kinetic studies of CH4 oxidation at conditions relevant to natural gas vehicles have quantified the thermodynamics and kinetics of competitive H2O adsorption and Pd(OH)2 formation, but none have addressed effects of H2O on O mobility.Catalysts2015-03-3152Review10.3390/catal50205615615942073-43442015-03-31doi: 10.3390/catal5020561Rahman GholamiMina AlyaniKevin Smithhttp://www.mdpi.com/2073-4344/5/2/534
This review is focused on the strategies and designs of magnetic nanostructured catalysts showing the enhanced and sustainable catalytic performances for the heterogeneous reduction of nitoaromatics. Magnetic catalysts have the benefits of easy recovery and reuse after the completion of the reactions and green chemical processes. Magnetic separation, among the various procedures for removing catalysts, not only obviates the requirement of catalyst filtration or centrifugation after the completion of reactions, but also provides a practical technique for recycling the magnetized nanostructured catalysts. Consequently, discussions will address the methodologies and exemplars for the reusable magnetic composite catalysts. Because the synthesis of ideal magnetic nanostructured catalysts is of primary importance in the development of high-quality sustainable processes, the designs, preparation methods and recyclability of various recoverable magnetic nanostructured catalysts are emphasized. The representative methods and strategies for the synthesis of durable and reusable magnetic nanostructured catalysts are highlighted. The advantages, disadvantages, recyclability and the efficiency of the introduced heterogeneous systems have been explored in the reduction of nitrobenzene derivatives.Catalysts2015-03-3152Review10.3390/catal50205345345602073-43442015-03-31doi: 10.3390/catal5020534Mohammadreza Shokouhimehrhttp://www.mdpi.com/2073-4344/5/2/518
A series of V- and/or Ti-loading MCM-41 catalysts are successfully synthesized with a hydrothermal method. The photocatalytic and thermal epoxidations of cyclohexene in the presence of tert-butyl hydroperoxide (t-BuOOH) were investigated with real-time monitored by NIR-Raman spectroscopy. It suggests that both V- and Ti-loading can be responsible for the cyclohexene epoxidation. Moreover, the complementary behavior of V- and Ti-loading may be related to a similar role of activation. Interestingly, the progress of the photo-epoxidation on V0.25Ti2/MCM-41 photocatalyst was monitored by changes in intensity of the characteristic Raman bands without interference from the UV-light irradiation. The result, for the first time, reveals that cyclohexene was directly photo-epoxidized to 1,2-epoxycyclohexane by t-BuOOH during the reaction. A possible mechanism of cyclohexene photo-epoxidation is also proposed for this study.Catalysts2015-03-3052Article10.3390/catal50205185185332073-43442015-03-30doi: 10.3390/catal5020518Hsiang-Yu ChanVan-Huy NguyenJeffrey WuVanesa Calvino-CasildaMiguel BañaresHsunling Baihttp://www.mdpi.com/2073-4344/5/2/500
This accounts introduces unique characteristics by adopting the acyclic diene metathesis (ADMET) polymerization for synthesis of conjugated polymers, poly(arylene vinylene)s, known as promising molecular electronics. The method is more suitable than the other methods in terms of atom efficiency affording defect-free, stereo-regular (exclusive trans) polymers with well-defined chain ends; the resultant polymers possess better property than those prepared by the conventional methods. The chain ends (vinyl group) in the resultant polymer prepared by ruthenium-carbene catalyst(s) can be modified by treating with molybdenum-alkylidene complex (olefin metathesis) followed by addition of various aldehyde (Wittig type cleavage), affording the end-functionalized polymers exclusively. An introduction of initiating fragment, the other conjugated segment, and one-pot synthesis of end-functionalized block copolymers, star shape polymers can be achieved by adopting this methodology.Catalysts2015-03-3052Review10.3390/catal50205005005172073-43442015-03-30doi: 10.3390/catal5020500Tahmina HaqueKotohiro Nomurahttp://www.mdpi.com/2073-4344/5/2/478
Deactivation of commercially relevant cobalt catalysts for Low Temperature Fischer-Tropsch (LTFT) synthesis is discussed with a focus on the two main long-term deactivation mechanisms proposed: Carbon deposits covering the catalytic surface and re-oxidation of the cobalt metal. There is a great variety in commercial, demonstration or pilot LTFT operations in terms of reactor systems employed, catalyst formulations and process conditions. Lack of sufficient data makes it difficult to correlate the deactivation mechanism with the actual process and catalyst design. It is well known that long term catalyst deactivation is sensitive to the conditions the actual catalyst experiences in the reactor. Therefore, great care should be taken during start-up, shutdown and upsets to monitor and control process variables such as reactant concentrations, pressure and temperature which greatly affect deactivation mechanism and rate. Nevertheless, evidence so far shows that carbon deposition is the main long-term deactivation mechanism for most LTFT operations. It is intriguing that some reports indicate a low deactivation rate for multi-channel micro-reactors. In situ rejuvenation and regeneration of Co catalysts are economically necessary for extending their life to several years. The review covers information from open sources, but with a particular focus on patent literature.Catalysts2015-03-2652Review10.3390/catal50204784784992073-43442015-03-26doi: 10.3390/catal5020478Erling RytterAnders Holmenhttp://www.mdpi.com/2073-4344/5/1/460
When using heteropolycompounds in the selective oxidation of isobutane to methacrolein and methacrylic acid, both the keeping of the primary structure (Keggin units) and the presence of acidic sites are necessary to obtain the desired products. The structural evolution of supported (NH4)3HPMo11VO40 (APMV) catalysts under preliminary thermal oxidizing and reducing treatments was investigated. Various techniques, such as TGA/DTG (Thermo-Gravimetric Analysis/Derivative Thermo-Gravimetry), H2-TPR (Temperature Programed Reduction), in situ XRD (X-Ray Diffraction) and XPS (X-ray Photoelectron Spectroscopy), were applied. It was clearly evidenced that the thermal stability and the reducibility of the Keggin units are improved by supporting 40% APMV active phase on Cs3PMo12O40 (CPM). The partial degradation of APMV takes place depending on temperature and reaction conditions. The decomposition of ammonium cations (releasing NH3) leads to the formation of vacancies favoring cationic exchanges between vanadium coming from the active phase and cesium coming from the support. In addition, the vanadium expelled from the Keggin structure is further reduced to V4+, species, which contributes (with Mo5+) to activate isobutane. The increase in reducibility of the supported catalyst is assumed to improve the catalytic performance in comparison with those of unsupported APMV.Catalysts2015-03-2351Article10.3390/catal50104604604772073-43442015-03-23doi: 10.3390/catal5010460Fangli JingBenjamin KatryniokElisabeth Bordes-RichardFranck DumeignilSébastien Paulhttp://www.mdpi.com/2073-4344/5/1/442
Cobalt nanostructures with different size and morphology, i.e., spherical nanoparticles, nanorods, and particles arranged into elongated structures, were prepared using micelles and microemulsions as confined reaction media. The syntheses were carried out using three types of systems: aqueous surfactant solutions, oil-in water (O/W), and water-in-oil (W/O) microemulsions. The influence of the surfactant and the precipitating agent used for synthesis was also investigated. For this purpose, cobalt nanostructures were prepared using different non-ionic surfactants, namely Synperonic® 10/6, Pluronic® P123 and a mixture of SPAN 20–TWEEN 80. Three different precipitating agents were used: sodium borohydride, sodium hydroxide, and oxalic acid. Our findings revealed that by changing the type of reaction media as well as the precipitating agent it is possible to modify the shape and size of the cobalt nanostructures. Moreover, the use of O/W microemulsion generates better results in terms of colloidal stability and uniformity of particle size with respect to W/O microemulsion. The different cobalt nanostructures were supported on commercial and mesoporous silica; transmission electron microscopy (TEM) images showed that after deposition the Co nanocrystals remain well dispersed on the silica supports. This behavior suggests their great potential in catalytic applications.Catalysts2015-03-1951Article10.3390/catal50104424424592073-43442015-03-19doi: 10.3390/catal5010442Gabriella CarloMatteo LualdiAnna VeneziaMagali BoutonnetMargarita Sanchez-Dominguezhttp://www.mdpi.com/2073-4344/5/1/424
Molybdenum carbide was supported on three types of carbon support—activated carbon; multi-walled carbon nanotubes; and carbon nanofibers—using ammonium molybdate and molybdic acid as Mo precursors. The use of activated carbon as support afforded an X-ray amorphous Mo phase, whereas crystalline molybdenum carbide phases were obtained on carbon nanofibers and, in some cases, on carbon nanotubes. When the resulting catalysts were tested in the hydrodeoxygenation (HDO) of guaiacol in dodecane, catechol and phenol were obtained as the main products, although in some instances significant amounts of cyclohexane were produced. The observation of catechol in all reaction mixtures suggests that guaiacol was converted into phenol via sequential demethylation and HDO, although the simultaneous occurrence of a direct demethoxylation pathway cannot be discounted. Catalysts based on carbon nanofibers generally afforded the highest yields of phenol; notably, the only crystalline phase detected in these samples was Mo2C or Mo2C-ζ, suggesting that crystalline Mo2C is particularly selective to phenol. At 350 °C, carbon nanofiber supported Mo2C afforded near quantitative guaiacol conversion, the selectivity to phenol approaching 50%. When guaiacol HDO was performed in the presence of acetic acid and furfural, guaiacol conversion decreased, although the selectivity to both catechol and phenol was increased.Catalysts2015-03-1651Article10.3390/catal50104244244412073-43442015-03-16doi: 10.3390/catal5010424Eduardo Santillan-JimenezMaxime PerduRobert PaceTonya MorganMark Crockerhttp://www.mdpi.com/2073-4344/5/1/406
We investigated the structural evolution of molybdenum carbides subjected to hot aqueous environments and their catalytic performance in low-temperature hydroprocessing of acetic acid. While bulk structures of Mo carbides were maintained after aging in hot liquid water, a portion of carbidic Mo sites were converted to oxidic sites. Water aging also induced changes to the non-carbidic carbon deposited during carbide synthesis and increased surface roughness, which in turn affected carbide pore volume and surface area. The extent of these structural changes was sensitive to the initial carbide structure and was lower under actual hydroprocessing conditions indicating the possibility of further improving the hydrothermal stability of Mo carbides by optimizing catalyst structure and operating conditions. Mo carbides were active in acetic acid conversion in the presence of liquid water, their activity being comparable to that of Ru/C. The results suggest that effective and inexpensive bio-oil hydroprocessing catalysts could be designed based on Mo carbides, although a more detailed understanding of the structure-performance relationships is needed, especially in upgrading of more complex reaction mixtures or real bio-oils.Catalysts2015-03-1351Article10.3390/catal50104064064232073-43442015-03-13doi: 10.3390/catal5010406Jae-Soon ChoiViviane SchwartzEduardo Santillan-JimenezMark CrockerSamuel LewisMichael LanceHarry MeyerKarren Morehttp://www.mdpi.com/2073-4344/5/1/392
Platinum catalysts supported on carbon xerogel and carbon black (Vulcan) were synthesized with the aim of investigating the influence of the characteristics of the support on the electrochemical performance of the catalysts. Three synthesis methods were compared: an impregnation method with two different reducing agents, sodium borohydride and formic acid, and a microemulsion method, in order to study the effect of the synthesis method on the physico-chemical properties of the catalysts. X-ray diffraction and transmission electron microscopy were applied. Cyclic voltammetry and chronoamperometry were used for studying carbon monoxide and methanol oxidation. Catalysts supported on carbon xerogel presented higher catalytic activities towards CO and CH3OH oxidation than catalysts supported on Vulcan. The higher mesoporosity of carbon xerogel was responsible for the favored diffusion of reagents towards catalytic centers.Catalysts2015-03-1351Article10.3390/catal50103923924052073-43442015-03-13doi: 10.3390/catal5010392Cinthia AlegreMaría GálvezRafael MolinerMaría Lázarohttp://www.mdpi.com/2073-4344/5/1/366
Perovskite oxides La1−xSrxCo1−yFeyO3–δ (LSCF) have been extensively investigated and developed as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs) due to mixed ionic–electronic conductivity and high electrooxygen reduction activity for oxygen reduction. Recent literature investigations show that cathode performances can be improved by metal surface modification or B-site substitution on LSCF. Although the specific reaction mechanism needs to be further investigated, the promoting effect of metal species in enhancing oxygen surface exchange and oxygen bulk diffusion is well recognized. To our knowledge, no previous reviews dealing with the effect of metal promotion on the cathodic performances of LSCF materials have been reported. In the present review, recent progresses on metal (Pd, Pt, Ag, Cu, Zn, Ni) promotion of LSCF are discussed focusing on two main aspects, the different synthesis approaches used (infiltration, deposition, solid state reaction, one pot citrate method) and the effects of metal promotion on structural properties, oxygen vacancies content and cathodic performances. The novelty of the work lies in the fact that the metal promotion at the B-site is discussed in detail, pointing at the effects produced by two different approaches, the LSCF surface modification by the metal or the metal ion substitution at the B-site of the perovskite. Moreover, for the first time in a review article, the importance of the combined effects of oxygen dissociation rate and interfacial oxygen transfer rate between the metal phase and the cathode phase is addressed for metal-promoted LSCF and compared with the un-promoted oxides. Perspectives on new research directions are shortly given in the conclusion.Catalysts2015-03-1251Review10.3390/catal50103663663912073-43442015-03-12doi: 10.3390/catal5010366Shaoli GuoHongjing WuFabrizio PuleoLeonarda Liottahttp://www.mdpi.com/2073-4344/5/1/349
We here report a computational approach on the mechanism of allylicamination reactions using allyl-alcohols and amines as the substrates and phosphoramidite palladium catalyst 1a, which operates in the presence of catalytic amount of 1,3-diethylurea as a co-catalyst. DFT calculations showed a cooperative hydrogen-bonding array between the urea moiety and the hydroxyl group of the allyl alcohol, which strengthens the hydrogen bond between the O-H moiety of the coordinated allyl-alcohol and the carbonyl-moiety of the ligand. This hydrogen bond pattern facilitates the (rate-limiting) C-O oxidative addition step and leads to lower energy isomers throughout the catalytic cycle, clarifying the role of the urea-moiety.Catalysts2015-03-1151Article10.3390/catal50103493493652073-43442015-03-11doi: 10.3390/catal5010349Yasemin GumrukcuBas de BruinJoost Reekhttp://www.mdpi.com/2073-4344/5/1/310
The oxygen reduction reaction (ORR) is the oldest studied and most challenging of the electrochemical reactions. Due to its sluggish kinetics, ORR became the major contemporary technological hurdle for electrochemists, as it hampers the commercialization of fuel cell (FC) technologies. Downsizing the metal particles to nanoscale introduces unexpected fundamental modifications compared to the corresponding bulk state. To address these fundamental issues, various synthetic routes have been developed in order to provide more versatile carbon-supported low platinum catalysts. Consequently, the approach of using nanocatalysts may overcome the drawbacks encountered in massive materials for energy conversion. This review paper aims at summarizing the recent important advances in carbon-supported metal nanoparticles preparation from colloidal methods (microemulsion, polyol, impregnation, Bromide Anion Exchange…) as cathode material in low temperature FCs. Special attention is devoted to the correlation of the structure of the nanoparticles and their catalytic properties. The influence of the synthesis method on the electrochemical properties of the resulting catalysts is also discussed. Emphasis on analyzing data from theoretical models to address the intrinsic and specific electrocatalytic properties, depending on the synthetic method, is incorporated throughout. The synthesis process-nanomaterials structure-catalytic activity relationships highlighted herein, provide ample new rational, convenient and straightforward strategies and guidelines toward more effective nanomaterials design for energy conversion.Catalysts2015-03-0651Review10.3390/catal50103103103482073-43442015-03-06doi: 10.3390/catal5010310Yaovi HoladeNihat SahinKarine ServatTeko NappornKouakou Kokohhttp://www.mdpi.com/2073-4344/5/1/298
In this work, the WGS performance of a conventional Ni/CeO2 bulk catalyst is compared to that of a carbon-supported Ni-CeO2 catalyst. The carbon-supported sample resulted to be much more active than the bulk one. The higher activity of the Ni-CeO2/C catalyst is associated to its oxygen storage capacity, a parameter that strongly influences the WGS behavior. The stability of the carbon-supported catalyst under realistic operation conditions is also a subject of this paper. In summary, our study represents an approach towards a new generation of Ni-ceria based catalyst for the pure hydrogen production via WGS. The dispersion of ceria nanoparticles on an activated carbon support drives to improved catalytic skills with a considerable reduction of the amount of ceria in the catalyst formulation.Catalysts2015-03-0651Article10.3390/catal50102982983092073-43442015-03-06doi: 10.3390/catal5010298Laura Pastor-PérezTomás ReinaSvetlana IvanovaMiguel CentenoJosé OdriozolaAntonio Sepúlveda-Escribanohttp://www.mdpi.com/2073-4344/5/1/286
An extra-heavy crude oil underground upgrading concept and laboratory experiments are presented which involve the addition of a hydrogen donor (tetralin) to an Orinoco Basin extra-heavy crude oil under steam injection conditions (280–315 °C and residence times of at least 24-h). Three iron-containing nanocatalysts (20 nm, 60 nm and 90 nm) were used and the results showed increases of up to 8° in API gravity, 26% desulfurization and 27% reduction in the asphaltene content of the upgraded product in comparison to the control reaction using inert sand. The iron nanocatalysts were characterized by SEM, XPS, EDAX, and Mössbauer spectroscopy before and after the upgrading reactions. The results indicated the presence of hematite (Fe2O3) as the predominant iron phase. The data showed that the catalysts were deactivating by particle sintering (~20% increase in particle size) and also by carbon deposition. Probable mechanisms of reactions are proposed. Catalysts2015-03-0551Article10.3390/catal50102862862972073-43442015-03-05doi: 10.3390/catal5010286Cesar OvallesVictor RiveroArelys Salazarhttp://www.mdpi.com/2073-4344/5/1/270
Composites consisting of carbon nanotubes (CNTs) grown directly on oxygen-deficient anatase TiO2 (TiO2−δ) were synthesized by a two-step chemical vapor deposition (CVD) method and applied in photocatalytic hydrogen production from aqueous methanol solutions using photodeposited Pt as the co-catalyst. Thermogravimetry coupled with mass spectroscopy, X-ray diffraction, scanning electron microscopy, photocurrent analysis, X-ray photoelectron spectroscopy, and (scanning) transmission electron microscopy were performed to investigate the physical and (photo)chemical properties of the synthesized CNT-TiO2−δ composites before and after photocatalytic methanol reforming. The initial photocatalytic activity of TiO2 was found to be significantly improved in the presence of oxygen vacancies. An optimized amount (~7.2 wt%) of CNTs grown on the TiO2−δ surface led to a highly effective stabilization of the photocatalytic performance of TiO2−δ, which is attributed to the improved dispersion and stability of the photodeposited Pt co-catalyst nanoparticles and enhanced separation efficiency of photogenerated electron-hole pairs, rendering the photocatalysts less prone to deactivation.Catalysts2015-03-0251Article10.3390/catal50102702702852073-43442015-03-02doi: 10.3390/catal5010270Peirong ChenLidong WangPing WangAleksander KostkaMichael WarkMartin MuhlerRadim Beranekhttp://www.mdpi.com/2073-4344/5/1/145
Deactivation of heterogeneous catalysts is a ubiquitous problem that causes loss of catalytic rate with time. This review on deactivation and regeneration of heterogeneous catalysts classifies deactivation by type (chemical, thermal, and mechanical) and by mechanism (poisoning, fouling, thermal degradation, vapor formation, vapor-solid and solid-solid reactions, and attrition/crushing). The key features and considerations for each of these deactivation types is reviewed in detail with reference to the latest literature reports in these areas. Two case studies on the deactivation mechanisms of catalysts used for cobalt Fischer-Tropsch and selective catalytic reduction are considered to provide additional depth in the topics of sintering, coking, poisoning, and fouling. Regeneration considerations and options are also briefly discussed for each deactivation mechanism.Catalysts2015-02-2651Review10.3390/catal50101451452692073-43442015-02-26doi: 10.3390/catal5010145Morris ArgyleCalvin Bartholomewhttp://www.mdpi.com/2073-4344/5/1/131
Bimetallic AuPd supported on different carbonaceous materials and TiO2 was tested in the liquid phase oxidation of commercial grade and raw glycerol. The latter was directly obtained from the base-catalyzed transesterification of edible rapeseed oil using KOH. The best catalytic results were obtained using activated carbon and nitrogen-functionalized carbon nanofibers as supports. In fact, the catalysts were more active using pure glycerol instead of the one obtained from rapeseed, where strong deactivation phenomena were present. Fourier transform infrared (FT-IR) and TEM were utilized to investigate the possible reasons for the observed loss of activity.Catalysts2015-02-1351Article10.3390/catal50101311311442073-43442015-02-13doi: 10.3390/catal5010131Carine Chan-ThawSebastiano CampisiDi WangLaura PratiAlberto Villahttp://www.mdpi.com/2073-4344/5/1/119
In this article, the combination of guanidine hydrochloride with co-catalyst ZnI2 proved to be a highly efficient heterogeneous catalyst for the environmentally benign, solvent-free synthesis of cyclic carbonates under mild reaction conditions. The effects of different co-catalysts as well as reaction parameters including catalyst loadings, CO2 pressure, reaction temperature, and reaction time on the coupling reaction of CO2 to propylene oxide were thoroughly investigated. With the molar ratio of guanidine hydrochloride to ZnI2 at 5:1, excellent yield (94%) and selectivity (≥99%) of propylene carbonate were obtained under 100 °C and at 1 MPa for 1.5 h. Additionally, ZnI2 could be recycled, but because of the washing loss of guanidine hydrochloride, there was a slight decrease in the yield of propylene carbonate. Gratifyingly, the activity of the catalytic system could be restored by adding additional 20 mol% of fresh guanidine hydrochloride, thus exhibiting excellent recyclability of the ZnI2 catalyst. Moreover, the binary catalysts were also versatile when using other epoxides for CO2 cycloaddition. A possible reaction mechanism was proposed wherein guanidine hydrochloride plays a dual role in activating CO2 and epoxide, and ZnI2 activated epoxide, simultaneously. The synergistic effect of guanidine hydrochloride and ZnI2 ensure the reaction proceeds effectively.Catalysts2015-02-1151Article10.3390/catal50101191191302073-43442015-02-11doi: 10.3390/catal5010119Bo LiuMengshuai LiuLin LiangJianmin Sunhttp://www.mdpi.com/2073-4344/5/1/106
The application of a catch-and-release system for soluble Pd species between water (reaction medium) and polystyrene (polymer support) was examined in the Suzuki coupling reaction with 2-bromothiophene and the Heck reaction with styrene or bromobenzene. Although a slight increase in particle size was observed by TEM after re-stabilization of the Pd species on linear polystyrene, no agglomeration was observed.Catalysts2015-02-0951Article10.3390/catal50101061061182073-43442015-02-09doi: 10.3390/catal5010106Atsushi OhtakaToshiyuki OkagakiGo HamasakaYasuhiro UozumiTsutomu ShinagawaOsamu ShimomuraRyôki Nomurahttp://www.mdpi.com/2073-4344/5/1/88
Pt-loaded β-type zeolite catalysts with constant Pt content (0.11 wt.%) and similar pore structure were prepared using a water-in-oil (w/o) microemulsion. The effect of Pt particle synthesis conditions using microemulsion (a type of Pt complex-forming agents and the molar ratio of complex-forming agent to Pt4+) on loaded Pt particle size was investigated. The Pt particle size of the Pt catalyst using tetraethylammonium chloride (TEAC) as a complex-forming agent with the molar TEAC/Pt ratio 10 was the minimum value (3.8 nm), and was much smaller than that (6.7 nm) prepared by the impregnation method. The utilization of the complex-forming agent of which hydrophobic groups occupied a small volume and the appropriate complex-forming agent/Pt ratio were favorable for synthesis of small Pt particles. The effect of loaded Pt particle size on the hydrocracking of the Fischer-Tropsch (FT) product was investigated using the Pt-loaded zeolite catalysts at 250 °C with an initial H2 pressure of 0.5 MPa, and reaction time of 1 h. The Pt catalyst with a Pt particle size of 4.2 nm prepared using the microemulsion exhibited the maximum corresponding jet fuel yield (30.0%), which was higher than that of the impregnated catalyst.Catalysts2015-02-0651Article10.3390/catal5010088881052073-43442015-02-06doi: 10.3390/catal5010088Toshiaki HanaokaTomohisa MiyazawaKatsuya ShimuraSatoshi Hiratahttp://www.mdpi.com/2073-4344/5/1/77
Water pollution from emerging contaminants (ECs) or emerging pollutants is an important environmental problem. Heterogeneous photocatalytic treatment, as advanced oxidation treatment of wastewater effluents, has been proposed to solve this problem. In this paper, a heterogeneous photocatalytic process was studied for emergent contaminants removal using paracetamol as a model contaminant molecule. TiO2 photocatalytic activity was evaluated using two photocatalytic reactor configurations: Photocatalyst solid suspension in wastewater in a stirred photoreactor and TiO2 supported on glass spheres (TGS) configuring a packed bed photoreactor. The surface morphology and texture of the TGS were monitored by scanning electron microscope (SEM). The influence of photocatalyst amount and wastewater pH were evaluated in the stirred photoreactor and the influence of wastewater flowrate was tested in the packed bed photoreactor, in order to obtain the optimal operation conditions. Moreover, results obtained were compared with those obtained from photolysis and adsorption studies, using the optimal operation conditions. Good photocatalytic activities have been observed and leads to the conclusion that the heterogeneous photocatalytic system in a packed bed is an effective method for removal of emerging pollutants.Catalysts2015-02-0251Article10.3390/catal501007777872073-43442015-02-02doi: 10.3390/catal5010077Mª BorgesDulce GarcíaTania HernándezJuan Ruiz-MoralesPedro Esparzahttp://www.mdpi.com/2073-4344/5/1/58
A series of Ni/Ce\(_{x}\)Zr\(_{1-x}\)O\(_{2}\)/SiO\(_{2}\) catalysts with different Zr/Ce mass ratios were prepared by incipient wetness impregnation. Ni/SiO\(_{2}\), Ni/CeO\(_{2}\) and Ni/ZrO\(_{2}\) were also prepared as reference materials to compare. Catalysts' performances were tested in ethanol steam reforming for hydrogen production and characterized by XRD, H\(_{2}\)-temperature programmed reduction (TPR), NH\(_{3}\)-temperature programmed desorption (TPD), TEM, ICP-AES and N\(_{2}\)-sorption measurements. The Ni/SiO\(_{2}\) catalyst led to a higher hydrogen selectivity than Ni/CeO\(_{2}\) and Ni/ZrO\(_{2}\), but it could not maintain complete ethanol conversion due to deactivation. The incorporation of Ce or Zr prior to Ni on the silica support resulted in catalysts with better performance for steam reforming, keeping complete ethanol conversion over time. When both Zr and Ce were incorporated into the catalyst, Ce\(_{x}\)Zr\(_{1-x}\)O\(_{2}\) solid solution was formed, as confirmed by XRD analyses. TPR results revealed stronger Ni-support interaction in the Ce\(_{x}\)Zr\(_{1-x}\)O\(_{2}\)-modified catalysts than in Ni/SiO\(_{2}\) one, which can be attributed to an increase of the dispersion of Ni species. All of the Ni/Ce\(_{x}\)Zr\(_{1-x}\)O\(_{2}\)/SiO\(_{2}\) catalysts exhibited good catalytic activity and stability after 8 h of time on stream at 600°. The best catalytic performance in terms of hydrogen selectivity was achieved when the Zr/Ce mass ratio was three.Catalysts2015-01-3051Article10.3390/catal501005858762073-43442015-01-30doi: 10.3390/catal5010058Jose CallesAlicia CarreroArturo VizcaínoMontaña Lindohttp://www.mdpi.com/2073-4344/5/1/40
The design of efficient catalytic layers of proton exchange membrane fuel cells (PEMFCs) requires the preparation of highly-loaded and highly-dispersed Pt/C catalysts. During the last few years, our work focused on the preparation of Pt/carbon xerogel electrocatalysts, starting from simple impregnation techniques that were further optimized via the strong electrostatic adsorption (SEA) method to reach high dispersion and a high metal weight fraction. The SEA method, which consists of the optimization of the precursor/support electrostatic impregnation through an adequate choice of the impregnation pH with regard to the support surface chemistry, leads to very well-dispersed Pt/C samples with a maximum 8 wt.% Pt after drying and reduction under H2. To increase the metal loading, the impregnation-drying-reduction cycle of the SEA method can be repeated several times, either with fresh Pt precursor solution or with the solution recycled from the previous cycle. In each case, a high dispersion (Pt particle size ~3 nm) is obtained. Finally, the procedure can be simplified by combination of the SEA technique with dry impregnation, leading to no Pt loss during the procedure.Catalysts2015-01-2851Review10.3390/catal501004040572073-43442015-01-28doi: 10.3390/catal5010040Nathalie JobStéphanie LambertAnthony ZubiaurChongjiang CaoJean-Paul Pirardhttp://www.mdpi.com/2073-4344/5/1/38
Palladium-catalyzed cross-coupling reactions constitute one of the most important and useful transformations in transition metal-catalyzed reactions [1–3]. Although this research field emerged in the 1970s, it is still attracting considerable attention in synthetic and organometallic chemistry. New catalysts and catalytic systems are being developed to further expand the utility of the reactions. This Special Issue on “Palladium Catalysts for Cross-Coupling Reaction” covers the diverse aspects of the ever-evolving field of palladium catalysts in cross coupling chemistry.[...]Catalysts2015-01-2251Editorial10.3390/catal501003838392073-43442015-01-22doi: 10.3390/catal5010038Kei Manabehttp://www.mdpi.com/2073-4344/5/1/26
The addition of Ru to Mn and Zr-modified Co/SiO2 catalysts, while applying different preparation orders and loading amounts, was investigated as a means of enhancing the Fischer-Tropsch synthesis reaction. The coimpregnation of Zr/SiO2 with Co, Mn and Ru gave the most attractive catalytic properties. This can be attributed to the higher dispersion of Co metal resulting from the coimpregnation of Co and Mn as well as enhanced reducibility due to the presence of Ru. The addition of a moderate amount of Ru together with the appropriate order of addition affected both the Co reducibility and the catalytic activity, primarily because of increased reducibility. The addition of even 0.1 wt.% Ru resulted in an obvious enhancement of Fischer-Tropsch synthesis activity.Catalysts2015-01-1251Article10.3390/catal501002626372073-43442015-01-12doi: 10.3390/catal5010026Tomohisa MiyazawaToshiaki HanaokaKatsuya ShimuraSatoshi Hiratahttp://www.mdpi.com/2073-4344/5/1/18
The continuous flow Suzuki-Miyaura reaction between various haloarenes and arylboronic acids was successfully achieved within only ca. 20 s during the single-pass through a cartridge filled with palladium on carbon (Pd/C). No palladium leaching was observed in the collected reaction solution by atomic absorption spectrometry (detection limit: 1 ppm).Catalysts2015-01-1251Article10.3390/catal501001818252073-43442015-01-12doi: 10.3390/catal5010018Tomohiro HattoriAya TsuboneYoshinari SawamaYasunari MonguchiHironao Sajikihttp://www.mdpi.com/2073-4344/5/1/3
In this concise review paper, we will address recent studies based on the generalized-gradient approximation (GGA) of the density functional theory (DFT) and on the periodic slab approach devoted to the understanding of the Fischer-Tropsch synthesis process on transition metal catalysts. As it will be seen, this computational combination arises as a very adequate strategy for the study of the reaction mechanisms on transition metal surfaces under well-controlled conditions and allows separating the influence of different parameters, e.g., catalyst surface morphology and coverage, influence of co-adsorbates, among others, in the global catalytic processes. In fact, the computational studies can now compete with research employing modern experimental techniques since very efficient parallel computer codes and powerful computers enable the investigation of more realistic molecular systems in terms of size and composition and to explore the complexity of the potential energy surfaces connecting reactants, to intermediates, to products of reaction. In the case of the Fischer-Tropsch process, the calculations were used to complement experimental work and to clarify the reaction mechanisms on different catalyst models, as well as the influence of additional components and co-adsorbate species in catalyst activity and selectivity.Catalysts2015-01-0951Review10.3390/catal50100033172073-43442015-01-09doi: 10.3390/catal5010003José FajínM. CordeiroJosé Gomeshttp://www.mdpi.com/2073-4344/5/1/1
The editors of Catalysts would like to express their sincere gratitude to the following reviewers for assessing manuscripts in 2014:[...]Catalysts2015-01-0851Editorial10.3390/catal5010001122073-43442015-01-08doi: 10.3390/catal5010001 Catalysts Editorial Officehttp://www.mdpi.com/2073-4344/4/4/414
A family of tungstated zirconia solid acid catalysts were synthesised via wet impregnation and subsequent thermochemical processing for the transformation of glucose to 5-hydroxymethylfurfural (HMF). Acid strength increased with tungsten loading and calcination temperature, associated with stabilisation of tetragonal zirconia. High tungsten dispersions of between 2 and 7 W atoms·nm−2 were obtained in all cases, equating to sub-monolayer coverages. Glucose isomerisation and subsequent dehydration via fructose to HMF increased with W loading and calcination temperature up to 600 °C, indicating that glucose conversion to fructose was favoured over weak Lewis acid and/or base sites associated with the zirconia support, while fructose dehydration and HMF formation was favoured over Brönsted acidic WOx clusters. Aqueous phase reforming of steam exploded rice straw hydrolysate and condensate was explored heterogeneously for the first time over a 10 wt% WZ catalyst, resulting in excellent HMF yields as high as 15% under mild reaction conditions.Catalysts2014-12-1544Article10.3390/catal40404144144262073-43442014-12-15doi: 10.3390/catal4040414Cao GiangAmin OsatiashtianiVannia dos SantosAdam LeeDavid WilsonKeith WaldronKaren Wilsonhttp://www.mdpi.com/2073-4344/4/4/397
Glycerol hydrodeoxygenation to 1,2-propanediol (1,2-PDO) is a reaction of high interest. However, the need for hydrogen supply is a main drawback of the process. According to the concept investigated here, 1,2-propanediol is efficiently formed using bio-glycerol feedstock with H2 formed in situ via ethanol aqueous phase reforming. Ethanol is thought to be a promising H2 source, as it is alcohol that can be used instead of methanol for transesterification of oils and fats. The H2 generated is consumed in the tandem reaction of glycerol hydrodeoxygenation. The reaction cycle proceeds in liquid phase at 220–250 °C and 1.5–3.5 MPa initial N2 pressure for a 2 and 4-h reaction time. Pt-, Ni- and Cu-based catalysts have been synthesized, characterized and evaluated in the reaction. Among the materials tested, Pt/Fe2O3-Al2O3 exhibited the most promising performance in terms of 1,2-propanediol productivity, while reusability tests showed a stable behavior. Structural integrity and no formation of carbonaceous deposits were verified via Temperature Programmed Desorption of hydrogen (TPD-H2) and thermogravimetric analysis of the fresh and used Pt/FeAl catalyst. A study on the effect of various operating conditions (reaction time, temperature and pressure) indicated that in order to maximize 1,2-propanediol productivity and yield, milder reaction conditions should be applied. The highest 1,2-propanediol yield, 53% (1.1 g1,2-PDO gcat−1·h−1), was achieved at a lower reaction temperature of 220 °C.Catalysts2014-12-1544Article10.3390/catal40403973974132073-43442014-12-15doi: 10.3390/catal4040397Efterpi VasiliadouAngeliki Lemonidouhttp://www.mdpi.com/2073-4344/4/4/375
Nanometer-sized materials have significantly different chemical and physical properties compared to bulk material. However, these properties do not only depend on the elemental composition but also on the structure, shape, size and arrangement. Hence, it is not only of great importance to develop synthesis routes that enable control over the final structure but also characterization strategies that verify the exact nature of the nanoparticles obtained. Here, we consider the verification of contemporary synthesis strategies for the preparation of bimetallic core-shell particles in particular in relation to potential particle structures, such as partial absence of core, alloying and raspberry-like surface. It is discussed what properties must be investigated in order to fully confirm a covering, pin-hole free shell and which characterization techniques can provide such information. Not uncommonly, characterization strategies of core-shell particles rely heavily on visual imaging like transmission electron microscopy. The strengths and weaknesses of various techniques based on scattering, diffraction, transmission and absorption for investigating core-shell particles are discussed and, in particular, cases where structural ambiguities still remain will be highlighted. Our main conclusion is that for particles with extremely thin or mono-layered shells—i.e., structures outside the limitation of most imaging techniques—other strategies, not involving spectroscopy or imaging, are to be employed. We will provide a specific example of Fe-Pt core-shell particles prepared in bicontinuous microemulsion and point out the difficulties that arise in the characterization process of such particles.Catalysts2014-11-2844Article10.3390/catal40403753753962073-43442014-11-28doi: 10.3390/catal4040375Emma WestssonGer Koperhttp://www.mdpi.com/2073-4344/4/4/356
Reducing the size from the bulk material to nanoparticles produces a scaling behavior in physical properties in the later ones, due to the large surface-to-volume fraction. By further size reduction, entering into the subnanometric cluster region, physical properties are largely affected by strong quantum confinement. These quantum size effects (HOMO-LUMO gap), the small size and the specific geometry award subnanometric clusters with totally new and fascinating properties, including cluster photoluminescence, enhanced catalytic activity, etc. In this review, we report an introduction to the physical properties of clusters based on the jellium model; the controlled synthesis by microemulsion methods and the catalytic properties in different areas as heterogeneous catalysis, photocatalysis or electrocatalysis among others.Catalysts2014-11-0344Review10.3390/catal40403563563742073-43442014-11-03doi: 10.3390/catal4040356David BucetaYolanda PiñeiroCarlos Vázquez-VázquezJosé RivasManuel López-Quintelahttp://www.mdpi.com/2073-4344/4/4/346
We present the chemical synthesis of hexaniobate nanotubes using two routes, (1) starting material K4Nb6O17 and (2) parent material of H4Nb6O17 via ion exchange. The as-synthesized materials were exfoliated by adjusting the pH to 9–10 using tetra-n-butylammonioum hydroxide (TBA+OH−), leading to a formation of hexaniobate nanotubes. In order to understand morphology a full characterization was conducted using SEM, HRTEM, BET and powder-XRD. The photocatalytic activity was evaluated using photolysis method using Bromocresol Green (BG) and Methyl Orange (MO) as model contaminants. Results indicate a nanotube porous oxide with large porous and surface area; the photocatalytic activity is about 95% efficient when comparing with commercial TiO2.Catalysts2014-10-2744Article10.3390/catal40403463463552073-43442014-10-27doi: 10.3390/catal4040346Maryam Zarei-ChaleshtoriVictor CorreaNatalie LópezManuel RamosRoya EdalatpourNancy RondeauRussell Chianellihttp://www.mdpi.com/2073-4344/4/3/321
In this review, we summarize our recent development of palladium(0)-catalyzed cross-coupling reactions of perfluoro organic compounds with organometallic reagents. The oxidative addition of a C–F bond of tetrafluoroethylene (TFE) to palladium(0) was promoted by the addition of lithium iodide, affording a trifluorovinyl palladium(II) iodide. Based on this finding, the first palladium-catalyzed cross-coupling reaction of TFE with diarylzinc was developed in the presence of lithium iodide, affording α,β,β-trifluorostyrene derivatives in excellent yield. This coupling reaction was expanded to the novel Pd(0)/PR3-catalyzed cross-coupling reaction of TFE with arylboronates. In this reaction, the trifluorovinyl palladium(II) fluoride was a key reaction intermediate that required neither an extraneous base to enhance the reactivity of organoboronates nor a Lewis acid additive to promote the oxidative addition of a C–F bond. In addition, our strategy utilizing the synergetic effect of Pd(0) and lithium iodide could be applied to the C–F bond cleavage of unreactive hexafluorobenzene (C6F6), leading to the first Pd(0)-catalyzed cross-coupling reaction of C6F6 with diarylzinc compounds.Catalysts2014-09-1043Review10.3390/catal40303213213452073-43442014-09-10doi: 10.3390/catal4030321Masato OhashiSensuke Ogoshihttp://www.mdpi.com/2073-4344/4/3/307
Pd-catalyzed, site-selective mono-cross-coupling of substrates with two identical halo groups is a useful method for synthesizing substituted monohalogenated arenes. Such arenes constitute an important class of compounds, which are commonly identified as drug components and synthetic intermediates. Traditionally, these site-selective reactions have been realized in a “substrate-controlled” manner, which is based on the steric and electronic differences between the two carbon-halogen bonds of the substrate. Recently, an alternative strategy, “catalyst-controlled” site-selective cross-coupling, has emerged. In this strategy, the preferred reaction site of a dihaloarene can be switched, merely by changing the catalyst used. This type of selective reaction further expands the utility of Pd-catalyzed cross-coupling. In this review, we summarize the reported examples of catalyst-controlled site-selectivity switching in Pd-catalyzed cross-coupling of dihaloarenes.Catalysts2014-08-2043Review10.3390/catal40303073073202073-43442014-08-20doi: 10.3390/catal4030307Kei ManabeMiyuki Yamaguchihttp://www.mdpi.com/2073-4344/4/3/305
Perovskite-type catalysts have been prominent oxide catalysts for many years due to attributes such as flexibility in choosing cations, significant thermal stability, and the unique nature of lattice oxygen. Nearly 90% metallic elements of the Periodic Table can be stabilized in perovskite’s crystalline framework [1]. Moreover, by following the Goldschmidt rule [2], the A- and/or B-site elements can be partially substituted, making perovskites extremely flexible in catalyst design. One successful example is the commercialization of noble metal-incorporated perovskites (e.g., LaFe0.57Co0.38Pd0.05O3) for automotive emission control used by Daihatsu Motor Co. Ltd. [3]. Thus, growing interest in, and application of perovskites in the fields of material sciences, heterogeneous catalysis, and energy storage have prompted this Special Issue on perovskite catalysts. [...]Catalysts2014-08-0743Editorial10.3390/catal40303053053062073-43442014-08-07doi: 10.3390/catal4030305Yu-Chuan LinKeith Hohnhttp://www.mdpi.com/2073-4344/4/3/299
Gold is an element that has fascinated mankind for millennia. The catalytic properties of gold have been a source of debate, due to its complete chemical inertness when in a bulk form, while it can oxidize CO at temperatures as low as ~200 K when in a nanocrystalline state, as discovered by Haruta in the late 1980s [1]. Since then, extensive activity in both applied and fundamental research on gold has been initiated. The importance of the catalysis by gold represents one of the fasted growing fields in science and is proven by the promising applications in several fields, such as green chemistry and environmental catalysis, in the synthesis of single-walled carbon nanotubes, as modifiers of Ni catalysts for methane steam and dry reforming reactions and in biological and electrochemistry applications. The range of reactions catalyzed by gold, as well as the suitability of different supports and the influence of the preparation conditions have been widely explored and optimized in applied research [2]. Gold catalysts appeared to be very different from the other noble metal-based catalysts, due to their marked dependence on the preparation method, which is crucial for the genesis of the catalytic activity. Several methods, including deposition-precipitation, chemical vapor deposition and cation adsorption, have been applied for the preparation of gold catalysts over reducible oxides, like TiO2. Among these methods, deposition-precipitation has been the most frequently employed method for Au loading, and it involves the use of tetrachloroauric (III) acid as a precursor. On the other hand, the number of articles dealing with Au-loaded acidic supports is smaller than that on basic supports, possibly because the deposition of [AuCl4]− or [AuOHxCl4−x]− species on acidic supports is difficult, due to their very low point of zero charge. Despite this challenge, several groups have reported the use of acidic zeolites as supports for gold. Zeolites are promising supports for Au stabilization, because of the presence of ion-exchange sites, such as NH4+, that can be substituted by Au+ ions through the elimination of NH4Cl [3]. Moreover, zeolites, due to their high thermal stability, the presence of a large surface area and micropores, may hinder Au sintering. [...]Catalysts2014-07-1643Editorial10.3390/catal40302992993042073-43442014-07-16doi: 10.3390/catal4030299Leonarda Liottahttp://www.mdpi.com/2073-4344/4/3/289
Indirect reductive amination of aldehydes, catalyzed by polymer supported triphenylphosphine-palladium acetate complex PS-TPP-Pd(OAc)2 catalyst have been developed. The imine is prepared with molecular sieves in the first stage, followed by reduction with potassium formate catalyzed by PS-TPP-Pd(OAc)2. The recovered catalyst could be reused for four consecutive cycles without loss in activity and provided an excellent yield of the desired products.Catalysts2014-07-1543Article10.3390/catal40302892892982073-43442014-07-15doi: 10.3390/catal4030289Sunil EkboteSandip GadgeBhalchandra Bhanagehttp://www.mdpi.com/2073-4344/4/3/276
Series catalysts of Ru-SnOx/Al2O3 with varying SnOx loading of 0–3 wt% were prepared, and their catalytic activity and selectivity have been discussed and compared for the selective hydrogenation of m-dinitrobenzene (m-DNB) to m-nitroaniline (m-NAN). The Ru-SnOx/Al2O3 catalysts were characterized by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and hydrogen temperature-programmed reduction (H2-TPR) and desorption (H2-TPD). Under the modification of SnOx, the reaction activity increased obviously, and the best selectivity to m-NAN reached above 97% at the complete conversion of m-DNB. With the increasing of the SnOx loading, the amount of active hydrogen adsorption on the surface of the catalyst increased according to the H2-TPD analysis, and the electron transferred from Ru to SnOx species, as determined by XPS, inducing an electron-deficient Ru, which is a benefit for the absorption of the nitro group. Therefore, the reaction rate and product selectivity were greatly enhanced. Moreover, the Ru-SnOx/Al2O3 catalyst presented high stability: it could be recycled four times without any loss in activity and selectivity.Catalysts2014-07-1443Article10.3390/catal40302762762882073-43442014-07-14doi: 10.3390/catal4030276Haiyang ChengWeiwei LinXiaoru LiChao ZhangFengyu Zhaohttp://www.mdpi.com/2073-4344/4/3/256
Water-in-oil (w/o) microemulsions were used as a template for the synthesis of mono- and bi-metallic nanoparticles. For that purpose, w/o-microemulsions containing H2PtCl6, H2PtCl6 + Pb(NO3)2 and H2PtCl6 + Bi(NO)3, respectively, were mixed with a w/o-microemulsion containing the reducing agent, NaBH4. The results revealed that it is possible to synthesize Pt, PtPb and PtBi nanoparticles of ~3–8 nm in diameter at temperatures of about 30°C. The catalytic properties of the bimetallic PtBi and PtPb nanoparticles were studied and compared with monometallic platinum nanoparticles. Firstly, the electrochemical oxidation of formic acid to carbon monoxide was investigated, and it was found that the resistance of the PtBi and PtPb nanoparticles against the catalyst-poisoning carbon monoxide was significantly higher compared to the Pt nanoparticles. Secondly, investigating the reduction of 4-nitrophenol to 4-aminophenol,we found that the bimetallic NPs are most active at 23 °C, while the order of the activity changes at higher temperatures, i.e., that the Pt nanoparticles are the most active ones at 36 and 49 °C. Furthermore, we observed a strong influence of the support, which was either a polymer or Al2O3. Thirdly, for the hydrogenation of allylbenzene to propylbenzene, the monometallic Pt NPs turned out to be the most active catalysts, followed by the PtPb and PtBi NPs. Comparing the two bimetallic nanoparticles, one sees that the PtPb NPs are significantly more active than the respective PtBi NPs.Catalysts2014-07-0143Article10.3390/catal40302562562752073-43442014-07-01doi: 10.3390/catal4030256Ramona KönigMichael SchwarzeReinhard SchomäckerCosima Stubenrauchhttp://www.mdpi.com/2073-4344/4/3/226
Automotive Three-Way Catalysts (TWC) were introduced more than 40 years ago. Despite that, the development of a sustainable TWC still remains a critical research topic owing to the increasingly stringent emission regulations together with the price and scarcity of precious metals. Among other material classes, perovskite-type oxides are known to be valuable alternatives to conventionally used TWC compositions and have demonstrated to be suitable for a wide range of automotive applications, ranging from TWC to Diesel Oxidation Catalysts (DOC), from NOx Storage Reduction catalysts (NSR) to soot combustion catalysts. The interest in these catalysts has been revitalized in the past ten years by the introduction of the concept of catalyst regenerability of perovskite-based TWC, which is in principle well applicable to other catalytic processes as well, and by the possibility to reduce the amounts of critical elements, such as precious metals without seriously lowering the catalytic performance. The aim of this review is to show that perovskite-type oxides have the potential to fulfil the requirements (high activity, stability, and possibility to be included into structured catalysts) for implementation in TWC.Catalysts2014-07-0143Review10.3390/catal40302262262552073-43442014-07-01doi: 10.3390/catal4030226Sylvain KeavSanthosh MatamDavide FerriAnke Weidenkaffhttp://www.mdpi.com/2073-4344/4/3/215
A series of arylalkylcarbinol derivatives were deracemized through sequential combination of Candida antarctica lipase B (CAL-B) catalyzed resolution by hydrolysis and Mitsunobu stereoinversion. The (S)-acetates were obtained in 71%–99% ee and 76%–89% yields. An enantiocomplementarity was established for the hydrolysis and acylation reactions with CAL-B lipase. Thus, the (S) and (R) enantiomers of Indan-1-yl acetate, 1,2,3,4-tetrahydro-1-naphthalenol acetate and 1-(2-naphthyl) ethyl acetate were obtained in 91%–99% ee and 76%–89% yield.Catalysts2014-06-2643Article10.3390/catal40302152152252073-43442014-06-26doi: 10.3390/catal4030215Nassima BouzemiIsmahane GribZahia HouieneLouisa Aribi-Zouiouechehttp://www.mdpi.com/2073-4344/4/2/196
The deactivation pattern of a “model” Ni/MgO catalyst in the pre-reforming of n-hexane with steam (T, 450 °C; P, 5–15 bar) is reviewed. The influence of the steam-to-carbon ratio (S/C, 1.5–3.5) on the rate of catalyst fouling by coking is ascertained. Catalyst fouling leads to an exponential decay in activity, denoting 1st-order dependence of the coking process on active sites availability. Hydrogen hinders the coking process, though slight activity decay is due to sintering of the active Ni phase. Deactivation by thiophene causes a sharp, almost linear, drop to nearly zero activity within only 6 h; this deactivation is likely due to dissociative adsorption of thiophene with subsequent strong, irreversible chemical adsorption of S-atoms on active Ni sites, i.e., irreversible poisoning. Modeling of activity decay curves (α, at/a0) by proper kinetic equations allows assessing the effects of temperature, pressure, S/C, H2 and thiophene feed on the deactivation pattern of the model Ni/MgO catalyst by coking, sintering, and poisoning phenomena.Catalysts2014-06-1942Review10.3390/catal40201961962142073-43442014-06-19doi: 10.3390/catal4020196Giuseppe TrunfioFrancesco Arenahttp://www.mdpi.com/2073-4344/4/2/186
The baker’s yeast mediated reduction of 2-acetyl-3-methyl sulfolane 1 to provide the corresponding alcohol 2 is described. Excellent efficiency and enantioselectivity (&amp;gt;98% ee) has been achieved under these mild environmentally benign reaction conditions. In direct contrast, the chemical reduction of 1 proceeds with poor yield (≤25%) and diastereocontrol.Catalysts2014-06-1842Article10.3390/catal40201861861952073-43442014-06-18doi: 10.3390/catal4020186Rebecca DeasyNoreen O'RiordanAnita Maguirehttp://www.mdpi.com/2073-4344/4/2/174
Conversion of arachidonic acid to prostaglandin G2/H2 catalyzed by prostaglandin H synthase (PGHS) is proposed to involve initial transfer of the C13 pro-(S) hydrogen atom from arachidonate to the Tyr385 radical in PGHS, followed by insertion of two oxygen molecules and several chemical bond rearrangements. The initial hydrogen-transfer was recently concluded to be a rate-limiting step in cyclooxygenase catalysis based on the observed intrinsic deuterium kinetic isotope effect values (Dkcat). In the present study, we have found that Dkcat values of both PGHS-1 and -2 show an unusual increase with temperatures in the range of 288–310 K, exhibiting an inverted temperature dependence. The value of lnDkcat, however, decreased linearly with 1/T, consistent with a typical Arrhenius relationship.Catalysts2014-06-1142Communication10.3390/catal40201741741852073-43442014-06-11doi: 10.3390/catal4020174Gang WuRichard KulmaczAh-Lim Tsaihttp://www.mdpi.com/2073-4344/4/2/162
Photocatalytic H2 evolution was examined using Pt-loaded TiO2-photocatalyst in the presence of amines as sacrificial agents. In the case of amines with all of the carbon attached to the hetero-atom such as 2-aminoethanol, 1,2-diamonoethane, 2-amino-1,3-propanediol, and 3-amino-1,2-propanediol, they were completely decomposed into CO2 and water to quantitatively evolve H2. On the other hand, the amines with both hetero-atoms and one methyl group at the β-positions (neighboring carbons) of amino group such as 2-amino-1-propanol and 1,2-diaminopropane were partially decomposed. Also, the photocatalytic H2 evolution using amines without the hetero-atoms at the β-positions such as ethylamine, propylamine, 1-butylamine, 1,3-diaminopropane, 2-propylamine, and 2-butylamine was inefficient. Thus, it was found that the neighboring hetero-atom strongly assisted the degradation of sacrificial amines. Moreover, rate constants for H2 evolution were compared among amines. In conclusion, the neighboring hetero-atom did not affect the rate constants but enhanced the yield of hydrogen evolution.Catalysts2014-05-2642Article10.3390/catal40201621621732073-43442014-05-26doi: 10.3390/catal4020162Masahide YasudaTakayuki TomoShoichi HirataTsutomu ShiragamiTomoko Matsumotohttp://www.mdpi.com/2073-4344/4/2/146
Direct methane solid oxide fuel cells, operated by supplying methane to a Ni/YSZ anode, suffer from degradation via accumulation of carbon deposits on the Ni surface. Coating a 40 µm thin film of La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) perovskite on the Ni/YSZ anode surface decreased the amount of carbon deposits, slowing down the degradation rate. The improvement in anode durability could be related to the oxidation activity of LSCF which facilitates oxidation of CH4 and carbon deposits. Analysis of the crystalline structure of LSCF revealed that LSCF was stable in the reducing anode environment under H2 and CH4 flow at 750 °C and retained its perovskite structure throughout the 475 h long-term stability test.Catalysts2014-05-0942Article10.3390/catal40201461461612073-43442014-05-09doi: 10.3390/catal4020146Jelvehnaz MirzababaeiSteven Chuanghttp://www.mdpi.com/2073-4344/4/2/129
Studying catalysts in situ is of high interest for understanding their surface structure and electronic states in operation. Herein, we present a study of epitaxial manganite perovskite thin films (Pr1−xCaxMnO3) active for the oxygen evolution reaction (OER) from electro-catalytic water splitting. X-ray absorption near-edge spectroscopy (XANES) at the Mn L- and O K-edges, as well as X-ray photoemission spectroscopy (XPS) of the O 1s and Ca 2p states have been performed in ultra-high vacuum and in water vapor under positive applied bias at room temperature. It is shown that under the oxidizing conditions of the OER a reduced Mn2+ species is generated at the catalyst surface. The Mn valence shift is accompanied by the formation of surface oxygen vacancies. Annealing of the catalysts in O2 atmosphere at 120 °C restores the virgin surfaces.Catalysts2014-04-2342Article10.3390/catal40201291291452073-43442014-04-23doi: 10.3390/catal4020129Daniel MierwaldtStephanie MildnerRosa ArrigoAxel Knop-GerickeEmanuel FrankeAndreas BlumensteinJörg HoffmannChristian Joosshttp://www.mdpi.com/2073-4344/4/2/116
The conversion of syngas (CO + H2) to gasoline-ranged hydrocarbons was carried out using a hybrid catalyst consisting of metal-loaded ZSM-5 coupled with Cu-ZnO in a near-critical n-hexane solvent. Methanol was synthesized from syngas over Cu-ZnO; subsequently, was converted to hydrocarbons through the formation of dimethyl ether (DME) over the metal-loaded ZSM-5. When 0.5 wt% Pd/ZSM-5 and 5 wt% Cu/ZSM-5 among the metal-loaded ZSM-5 catalysts with Pd, Co, Fe or Cu were employed as a portion of the hybrid catalyst, the gasoline-ranged hydrocarbons were selectively produced (the gasoline-ranged hydrocarbons in all hydrocarbons: 59% for the hybrid catalyst with Pd/ZSM-5 and 64% for that with Cu/ZSM-5) with a similar CO conversion during the reaction. An increase in the Cu loading on ZSM-5 resulted in increasing the yield of the gasoline-ranged hydrocarbons, and in decreasing the yield of DME. Furthermore, the hybrid catalyst with Cu/ZSM-5 exhibited no deactivation for 30 h of the reaction. It was revealed that a hybrid catalyst containing Cu/ZSM-5 was efficient in the selective synthesis of gasoline-ranged hydrocarbons from syngas via methanol in the near-critical n-hexane fluid.Catalysts2014-04-2342Article10.3390/catal40201161161282073-43442014-04-23doi: 10.3390/catal4020116Ting MaHiroyuki ImaiManami YamawakiKazusa TerasakaXiaohong Lihttp://www.mdpi.com/2073-4344/4/2/89
Benzyl alcohol oxidation was carried out in toluene as solvent, in the presence of the potentially inhibiting oxidation products benzaldehyde and benzoic acid. Benzoic acid, or a product of benzoic acid, is identified to be the inhibiting species. The presence of a basic potassium salt (K2CO3 or KF) suppresses this inhibition, but promotes the formation of benzyl benzoate from the alcohol and aldehyde. When a small amount of water is added together with the potassium salt, an even greater beneficial effect is observed, due to a synergistic effect with the base. A kinetic model, based on the three main reactions and four major reaction components, is presented to describe the concentration-time profiles and inhibition. The inhibition, as well as the effect of the base, was captured in the kinetic model, by combining strong benzoic acid adsorption and competitive adsorption with benzyl alcohol. The effect of the potassium salt is accounted for in terms of neutralization of benzoic acid.Catalysts2014-04-1542Article10.3390/catal4020089891152073-43442014-04-15doi: 10.3390/catal4020089Emmanuel SkupienRob BergerVera SantosJorge GasconMichiel MakkeeMichiel KreutzerPatricia KooymanJacob MoulijnFreek Kapteijnhttp://www.mdpi.com/2073-4344/4/2/77
In this study, we presented the synthesis of LaNi0.3Co0.7O3−δ (LNCO) and SrCo0.8Fe0.2O3−δ (SFCO) perovskites and their catalytic reactivity under CO gas flow. The synthesis method is based on the complexation method combining ethylenediaminetetraacetic acid (EDTA)-citrate. The as-prepared materials were characterized using X-ray diffraction/Rietveld refinement and electron microscopy. The diffractograms and Rietveld refinement showed that the EDTA-citrate method allows us to obtain monophasic powders with submicronic size. The structural analyses revealed different morphologies linked to the crystallinity of LNCO and SFCO, and to the mean crystallite size. The catalytic performances of LNCO and SFCO were studied in situ by Fourier Transform InfraRed spectrometer as a function of time and temperature. The catalytic process gave rise to total oxidation with carbon dioxide and water production. LNCO and SFCO exhibit the same profile for catalytic activity with a conversion rate of twice as high for LNCO. Below 150 °C, the kinetic conversion is slow, but beyond this temperature they reach rapidly the complete transformation at 250 °C and 275 °C for LNCO and SFCO, respectively.Catalysts2014-03-3142Article10.3390/catal402007777882073-43442014-03-31doi: 10.3390/catal4020077Andarair dos SantosMadjid ArabLoic PatoutCarlson de Souzahttp://www.mdpi.com/2073-4344/4/1/49
This focused review article underscores how metal reduction promoters can impact deactivation phenomena associated with cobalt Fischer-Tropsch synthesis catalysts. Promoters can exacerbate sintering if the additional cobalt metal clusters, formed as a result of the promoting effect, are in close proximity at the nanoscale to other cobalt particles on the surface. Recent efforts have shown that when promoters are used to facilitate the reduction of small crystallites with the aim of increasing surface Co0 site densities (e.g., in research catalysts), ultra-small crystallites (e.g., &amp;lt;2–4.4 nm) formed are more susceptible to oxidation at high conversion relative to larger ones. The choice of promoter is important, as certain metals (e.g., Au) that promote cobalt oxide reduction can separate from cobalt during oxidation-reduction (regeneration) cycles. Finally, some elements have been identified to promote reduction but either poison the surface of Co0 (e.g., Cu), or produce excessive light gas selectivity (e.g., Cu and Pd, or Au at high loading). Computational studies indicate that certain promoters may inhibit polymeric C formation by hindering C-C coupling.Catalysts2014-03-1141Review10.3390/catal401004949762073-43442014-03-11doi: 10.3390/catal4010049Gary JacobsWenping MaBurtron Davis